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

@@ -6041,20 +6041,20 @@ 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 $CXX supports __attribute__((warn_unused))])
-AC_LANG_PUSH([C++])
-save_CXXFLAGS=$CXXFLAGS
-CXXFLAGS="$CXXFLAGS -Werror -Wunknown-pragmas"
-AC_COMPILE_IFELSE([AC_LANG_SOURCE([
+    AC_MSG_CHECKING([whether $CXX supports __attribute__((warn_unused))])
+    AC_LANG_PUSH([C++])
+    save_CXXFLAGS=$CXXFLAGS
+    CXXFLAGS="$CXXFLAGS -Werror -Wunknown-pragmas"
+    AC_COMPILE_IFELSE([AC_LANG_SOURCE([
             struct __attribute__((warn_unused)) dummy {};
         ])], [
             AC_DEFINE([HAVE_GCC_ATTRIBUTE_WARN_UNUSED],[1])
             AC_MSG_RESULT([yes])
         ], [AC_MSG_RESULT([no])])
-CXXFLAGS=$save_CXXFLAGS
+    CXXFLAGS=$save_CXXFLAGS
 AC_LANG_POP([C++])
+fi
 
 AC_SUBST(HAVE_GCC_AVX)
 AC_SUBST(HAVE_GCC_STACK_PROTECTOR_STRONG)