build: don't pass -fno-builtin unless it works

Previously, we passed it whenever the compiler wasn't MSVC.  PGI,
however, doesn't support -fno-builtin (and isn't MSVC).  According
to PGI's documentation --no_builtin should work, but in practice
that doesn't seem to be the case.  pgc++ accepts --builtin but not
--no_builtin, pgcc doesn't take either.

CMakeLists.txt

@@ -80,7 +80,8 @@ if (MSVC)
 endif (MSVC)
 
 # https://cmake.org/Bug/view.php?id=8246
-if(NOT MSVC)
+check_c_compiler_flag("-fno-builtin" CFLAG_fno_builtin)
+if(CFLAG_fno_builtin)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-builtin")
 endif()