configure: Support disabling warnings in $gcc_flags

-Wall enables a bunch of warnings at once.  configure puts it after
$gcc_flags.  This makes it impossible to disable warnings enabled by
-Wall there.  Fix by putting configured flags last.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

configure

@@ -154,7 +154,7 @@ int main(void) { return 0; }
 EOF
 for flag in $gcc_flags; do
     if compile_prog "-Werror $QEMU_CFLAGS" "-Werror $flag" ; then
-	QEMU_CFLAGS="$flag $QEMU_CFLAGS"
+	QEMU_CFLAGS="$QEMU_CFLAGS $flag"
     fi
 done