Don't check HAVE_GCC_ATTRIBUTE_WARN_UNUSED on MSVC

...where -Werror -Wunknown-pragmas command line arguments will trivially make
the test always fail, anyway

Change-Id: Iae5534054331d41508f39d5cf141803ab5bc8ba7

configure.ac

@@ -6041,7 +6041,6 @@ if test "$GCC" = "yes" -o "$COM_IS_CLANG" = TRUE; then
             AC_MSG_RESULT([yes])])
     CXXFLAGS=$save_CXXFLAGS
     AC_LANG_POP([C++])
-fi
 
     AC_MSG_CHECKING([whether $CXX supports __attribute__((warn_unused))])
     AC_LANG_PUSH([C++])
@@ -6055,6 +6054,7 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([
         ], [AC_MSG_RESULT([no])])
     CXXFLAGS=$save_CXXFLAGS
 AC_LANG_POP([C++])
+fi
 
 AC_SUBST(HAVE_GCC_AVX)
 AC_SUBST(HAVE_GCC_STACK_PROTECTOR_STRONG)