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.

diff --git a/configure.ac b/configure.ac
index 0fd8c1e4b2640e9f48c7a9929669904bcf40ffe4..2e7f97b2fc84f54510b54f6c268ea0ac564453a8 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -588,8 +588,9 @@ if test "$GCC" = yes ; then
        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])
                ], [