build: begin working on supporting PGI

PGI doesn't actually work yet; there are a few things to fix in Squash:

 * The splicing code relies on TLS, but PGI doesn't support it (!).
   It should be possible to tweak the code a bit to avoid TLS, which
   would be a good thing for portability in general; TCC doesn't
   support TLS either, though PGI is the only modern non-toy compiler
   I'm aware of which doesn't.
 * PGI doesn't like -fno-builtin, and it's not MSVC.

There also may be something we can fix in Hedley; I was getting an
error from the conformant array parameters (HEDLEY_ARRAY_PARAM).  It
should work in any C99-compliant compiler, but the error was
sufficiently cryptic that I want to take a closer look before pushing
a change.

Additionally, there are issues in libaries various plugins use.  IIRC
brotli, gipfeli, and libdeflate (and maybe others) have issues I
haven't looked at yet.

If anyone wants to take a stab, there is now a community edition:

  http://www.pgroup.com/products/community.htm

I'll try to get back to it soon (I'm curious about performance).  IIRC
brotli, gipfeli, libdeflate, and possibly others will need some
changes.

cmake/RequireStandard.cmake

@@ -1,7 +1,20 @@
 include (AddCompilerFlags)
 
 function (target_require_c_standard target standard)
-  if (${CMAKE_VERSION} VERSION_LESS 3.1 OR "${CMAKE_C_COMPILER_ID}" STREQUAL "Intel")
+  if ("${CMAKE_C_COMPILER_ID}" STREQUAL "PGI")
+    get_target_property (sources ${target} SOURCES)
+    if ("${standard}" STREQUAL "c90")
+      set(standard "c89")
+    endif ()
+
+    foreach (source ${sources})
+      if ("${source}" MATCHES "\\.c$")
+        source_file_add_compiler_flags ("${source}" "-${standard}")
+      endif ()
+    endforeach (source)
+
+    unset (sources)
+  elseif (${CMAKE_VERSION} VERSION_LESS 3.1 OR "${CMAKE_C_COMPILER_ID}" STREQUAL "Intel")
     get_target_property (sources ${target} SOURCES)
 
     foreach (source ${sources})
@@ -29,7 +42,17 @@ function (target_require_c_standard target standard)
 endfunction ()
 
 function (target_require_cxx_standard target standard)
-  if (${CMAKE_VERSION} VERSION_LESS 3.1 OR "${CMAKE_CXX_COMPILER_ID}" STREQUAL "Intel")
+  if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "PGI")
+    get_target_property (sources ${target} SOURCES)
+
+    foreach (source ${sources})
+      if ("${source}" MATCHES "\\.(cpp|cc|cxx)$")
+        source_file_add_compiler_flags ("${source}" "-${standard}")
+      endif ()
+    endforeach (source)
+
+    unset (sources)
+  elseif (${CMAKE_VERSION} VERSION_LESS 3.1 OR "${CMAKE_CXX_COMPILER_ID}" STREQUAL "Intel")
     get_target_property (sources ${target} SOURCES)
 
     foreach (source ${sources})