build: fix detecting support for -Wno-* compiler flags

Since GCC 4.4, gcc does not warn about unknown -Wno-* flags. At
least, it does not warning unless another warning is raised as well
(https://gcc.gnu.org/wiki/FAQ#wnowarning).

We didn't notice up to now, because we only tested flags that GCC
actually supports.

Hack around this, by checking for the -W* counterpart instead.

m4/compiler_warnings.m4

@@ -30,7 +30,8 @@ if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
 		      -Wundef -Wimplicit-function-declaration \
 		      -Wpointer-arith -Winit-self \
 		      -Wmissing-include-dirs; do
-		CFLAGS="$CFLAGS_MORE_WARNINGS $CFLAGS_EXTRA $option $CFLAGS_SAVED"
+		dnl GCC 4.4 does not warn when checking for -Wno-* flags (https://gcc.gnu.org/wiki/FAQ#wnowarning)
+		CFLAGS="$CFLAGS_MORE_WARNINGS $CFLAGS_EXTRA $(printf '%s' "$option" | sed 's/^-Wno-/-W/')  $CFLAGS_SAVED"
 		AC_MSG_CHECKING([whether gcc understands $option])
 		AC_TRY_COMPILE([], [],
 			has_option=yes,