build ncnn error with master

[onexuan]

In file included from /usr/local/include/google/protobuf/text_format.h:48:
In file included from /usr/local/include/google/protobuf/message.h:119:
/usr/local/include/google/protobuf/message_lite.h:117:3: error: unknown type
name 'constexpr'
constexpr const T& get() const { return reinterpret_cast<const T&>(union_); }
^
/usr/local/include/google/protobuf/message_lite.h:117:13: error: expected member
name or ';' after declaration specifiers
constexpr const T& get() const { return reinterpret_cast<const T&>(union_); }

/usr/local/include/google/protobuf/message_lite.h:110:11: error: use of
    undeclared identifier 'union_'
  new (&union_) T();
        ^
/usr/local/include/google/protobuf/message_lite.h:114:5: error: use of
    undeclared identifier 'get_mutable'
  get_mutable()->~T();
  ^
/usr/local/include/google/protobuf/message_lite.h:131:48: error: found '<::'
    after a template name which forms the digraph '<:' (aka '[') and a ':',
    did you mean '< ::'?
LIBPROTOBUF_EXPORT extern ExplicitlyConstructed<::std::string> fixed_add...
                                             ^~~
                                             < ::
/usr/local/include/google/protobuf/message_lite.h:137:37: error: no member named
    'get' in
    'google::protobuf::internal::ExplicitlyConstructed<std::__1::basic_string<char>
    >'
return fixed_address_empty_string.get();
       ~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
In file included from /Users/yang/Downloads/ncnn-master/tools/caffe/caffe2ncnn.cpp:27:
In file included from /usr/local/include/google/protobuf/text_format.h:48:
/usr/local/include/google/protobuf/message.h:345:51: warning: 'final' keyword is
    a C++11 extension [-Wc++11-extensions]
virtual const Reflection* GetReflection() const final {
                                                ^
In file included from /Users/yang/Downloads/ncnn-master/tools/caffe/caffe2ncnn.cpp:30:
In file included from /Users/yang/Downloads/ncnn-master/build-mac/tools/caffe/caffe.pb.h:24:
/usr/local/include/google/protobuf/arenastring.h:175:51: warning: unused
    parameter 'default_value' [-Wunused-parameter]
    ArenaStringPtr* other, const ::std::string* default_value, Arena* arena) {
                                                ^
/usr/local/include/google/protobuf/arenastring.h:175:73: warning: unused
    parameter 'arena' [-Wunused-parameter]
    ArenaStringPtr* other, const ::std::string* default_value, Arena* arena) {
                                                                      ^
In file included from /Users/yang/Downloads/ncnn-master/tools/caffe/caffe2ncnn.cpp:30:
In file included from /Users/yang/Downloads/ncnn-master/build-mac/tools/caffe/caffe.pb.h:25:
In file included from /usr/local/include/google/protobuf/generated_message_table_driven.h:34:
In file included from /usr/local/include/google/protobuf/map.h:49:
In file included from /usr/local/include/google/protobuf/map_type_handler.h:35:
In file included from /usr/local/include/google/protobuf/wire_format_lite_inl.h:44:
/usr/local/include/google/protobuf/repeated_field.h:112:21: warning: 'final'
    keyword is a C++11 extension [-Wc++11-extensions]
class RepeatedField final {
                  ^
/usr/local/include/google/protobuf/repeated_field.h:123:30: warning: rvalue
    references are a C++11 extension [-Wc++11-extensions]
RepeatedField(RepeatedField&& other) noexcept;
                           ^
/usr/local/include/google/protobuf/repeated_field.h:123:39: error: expected ';'
    at end of declaration list
RepeatedField(RepeatedField&& other) noexcept;
                                    ^
                                    ;
/usr/local/include/google/protobuf/repeated_field.h:124:41: warning: rvalue
    references are a C++11 extension [-Wc++11-extensions]
RepeatedField& operator=(RepeatedField&& other) noexcept;
                                      ^
/usr/local/include/google/protobuf/repeated_field.h:124:50: error: expected ';'
    at end of declaration list
RepeatedField& operator=(RepeatedField&& other) noexcept;
                                               ^
                                               ;
/usr/local/include/google/protobuf/repeated_field.h:468:38: warning: rvalue
    references are a C++11 extension [-Wc++11-extensions]
void Add(typename TypeHandler::Type&& value,
                                   ^
/usr/local/include/google/protobuf/repeated_field.h:736:49: warning: rvalue
    references are a C++11 extension [-Wc++11-extensions]
static inline string* New(Arena* arena, string&& value) {
                                              ^
/usr/local/include/google/protobuf/repeated_field.h:766:24: warning: 'final'
    keyword is a C++11 extension [-Wc++11-extensions]
class RepeatedPtrField final : private internal::RepeatedPtrFieldBase {
                     ^
/usr/local/include/google/protobuf/repeated_field.h:778:36: warning: rvalue
    references are a C++11 extension [-Wc++11-extensions]
RepeatedPtrField(RepeatedPtrField&& other) noexcept;
                                 ^
/usr/local/include/google/protobuf/repeated_field.h:778:45: error: expected ';'
    at end of declaration list
RepeatedPtrField(RepeatedPtrField&& other) noexcept;
                                          ^
                                          ;
/usr/local/include/google/protobuf/repeated_field.h:779:47: warning: rvalue
    references are a C++11 extension [-Wc++11-extensions]
RepeatedPtrField& operator=(RepeatedPtrField&& other) noexcept;
                                            ^
/usr/local/include/google/protobuf/repeated_field.h:779:56: error: expected ';'
    at end of declaration list
RepeatedPtrField& operator=(RepeatedPtrField&& other) noexcept;
                                                     ^
                                                     ;
/usr/local/include/google/protobuf/repeated_field.h:787:19: warning: rvalue
    references are a C++11 extension [-Wc++11-extensions]
void Add(Element&& value);
                ^
/usr/local/include/google/protobuf/repeated_field.h:1090:59: warning: rvalue
    references are a C++11 extension [-Wc++11-extensions]
inline RepeatedField<Element>::RepeatedField(RepeatedField&& other) noexcept
                                                        ^
/usr/local/include/google/protobuf/repeated_field.h:1090:32: error: out-of-line
    declaration of a member must be a definition [-Wout-of-line-declaration]
inline RepeatedField<Element>::RepeatedField(RepeatedField&& other) noexcept
     ~~~~~~~~~~~~~~~~~~~~~~~~^
/usr/local/include/google/protobuf/repeated_field.h:1090:68: error: expected ';'
    at end of declaration
inline RepeatedField<Element>::RepeatedField(RepeatedField&& other) noexcept
                                                                 ^
                                                                 ;
/usr/local/include/google/protobuf/repeated_field.h:1090:69: error: unknown type
    name 'noexcept'
inline RepeatedField<Element>::RepeatedField(RepeatedField&& other) noexcept
                                                                  ^
/usr/local/include/google/protobuf/repeated_field.h:1091:5: error: expected
    unqualified-id
  : RepeatedField() {
  ^
/usr/local/include/google/protobuf/repeated_field.h:1103:18: warning: rvalue
    references are a C++11 extension [-Wc++11-extensions]
  RepeatedField&& other) noexcept {
               ^
/usr/local/include/google/protobuf/repeated_field.h:1102:56: error: out-of-line
    declaration of a member must be a definition [-Wout-of-line-declaration]
inline RepeatedField<Element>& RepeatedField<Element>::operator=(
                             ~~~~~~~~~~~~~~~~~~~~~~~~^
/usr/local/include/google/protobuf/repeated_field.h:1103:27: error: expected ';'
    at end of declaration
  RepeatedField&& other) noexcept {
                        ^
                        ;
/usr/local/include/google/protobuf/repeated_field.h:1103:28: error: C++ requires
    a type specifier for all declarations
  RepeatedField&& other) noexcept {
                         ^
/usr/local/include/google/protobuf/repeated_field.h:1103:36: error: expected ';'
    after top level declarator
  RepeatedField&& other) noexcept {
                                 ^
                                 ;
/usr/local/include/google/protobuf/repeated_field.h:1928:21: warning: rvalue
    references are a C++11 extension [-Wc++11-extensions]
  RepeatedPtrField&& other) noexcept
                  ^
/usr/local/include/google/protobuf/repeated_field.h:1927:35: error: out-of-line
    declaration of a member must be a definition [-Wout-of-line-declaration]
inline RepeatedPtrField<Element>::RepeatedPtrField(
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
49 warnings and 20 errors generated.
make[2]: *** [tools/caffe/CMakeFiles/caffe2ncnn.dir/caffe2ncnn.cpp.o] Error 1
make[1]: *** [tools/caffe/CMakeFiles/caffe2ncnn.dir/all] Error 2
make: *** [all] Error 2

[onexuan]

Fix
install 3.1.0 protobuf