Fix build issues with gcc 9 (#1090)

* Remove redundant moves in return statements

With recent g++ versions:

../include/bm/bm_sim/queueing.h:780:24: error: redundant move in return statement [-Werror=redundant-move]
  780 |     return std::move(fn);
      |                        ^
../include/bm/bm_sim/queueing.h:780:24: note: remove 'std::move' call
cc1plus: all warnings being treated as error

While copy elision is not possible when retruning a function parameter,
if the other conditions for NVO are met, a move operation should be
used. The std::move doesn't cause the compiler to perform worse (unlike
for RVO), but it is redundant.

* Do not treat deprecated-declarations warnings as errors

When compiling code which depends on the generated Protobuf code for
P4Runtime.

* Fix linking for example program in simple_switch_grpc/tests

Linking was broken after Ubuntu 20.04 / gcc 9 upgrade.
Not sure exactly why but removing some unused library dependencies can
resolve the issue ("undefined reference" for PI symbols).

include/bm/bm_sim/queueing.h

@@ -777,15 +777,15 @@ class QueueingLogicPriRL {
   Function for_each_q(size_t queue_id, Function fn) {
     auto &q_info = get_queue(queue_id);
     for (auto &q_info_pri : q_info) fn(q_info_pri);
-    return std::move(fn);
+    return fn;
   }
 
   template <typename Function>
   Function for_one_q(size_t queue_id, size_t priority, Function fn) {
     auto &q_info = get_queue(queue_id);
     auto &q_info_pri = q_info.at(priority);
     fn(q_info_pri);
-    return std::move(fn);
+    return fn;
   }
 
   struct SetCapacityFn {

targets/simple_switch_grpc/tests/Makefile.am

@@ -3,6 +3,9 @@ ACLOCAL_AMFLAGS = ${ACLOCAL_FLAGS} -I m4
 # Ensures that gtest has been built
 SUBDIRS = ../../../third_party
 
+# for deprecated Protobuf fields
+AM_CXXFLAGS += -Wno-error=deprecated-declarations
+
 AM_CPPFLAGS += \
 -isystem $(top_srcdir)/../../third_party/gtest/include \
 -I$(top_srcdir) \
@@ -31,6 +34,7 @@ check_PROGRAMS = example test_gtest
 common_source = utils.h utils.cpp base_test.h base_test.cpp
 
 example_SOURCES = example.cpp utils.h utils.cpp
+
 test_gtest_SOURCES = \
 $(common_source) main.cpp \
 test_basic.cpp test_grpc_dp.cpp test_packet_io.cpp \
@@ -45,7 +49,11 @@ if WITH_SYSREPO
 test_gtest_SOURCES += test_gnmi.cpp
 endif
 
-LDADD = \
+example_LDADD = \
+-lpiprotogrpc -lpiprotobuf \
+$(PROTOBUF_LIBS) $(GRPC_LIBS)
+
+test_gtest_LDADD = \
 $(top_builddir)/libsimple_switch_grpc.la \
 $(top_builddir)/../simple_switch/libsimpleswitch.la \
 -lpiprotogrpc -lpiprotobuf \