Build: Detect supported compiler warning flags better.

Clang and nowadays also GCC accept any -Wfoobar option
but then may give a warning that an unknown warning option
was specified. To avoid adding unsupported warning options,
the options are now tested with -Werror.

Thanks to Charles Diza.

configure.ac

diff --git a/configure.ac b/configure.ac
index 2df6797..e41979b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -749,8 +749,9 @@
 	do
 		AC_MSG_CHECKING([if $CC accepts $NEW_FLAG])
 		OLD_CFLAGS="$CFLAGS"
-		CFLAGS="$CFLAGS $NEW_FLAG"
-		AC_COMPILE_IFELSE([AC_LANG_SOURCE([void foo(void) { }])], [
+		CFLAGS="$CFLAGS $NEW_FLAG -Werror"
+		AC_COMPILE_IFELSE([AC_LANG_SOURCE(
+				[void foo(void); void foo(void) { }])], [
 			AM_CFLAGS="$AM_CFLAGS $NEW_FLAG"
 			AC_MSG_RESULT([yes])
 		], [