configure: Modify detection of supported warning options

Reversing the order of the warning options and -Werror is important
when clang is used instead of gcc. It changes nothing for gcc.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -1105,7 +1105,7 @@ cat > $TMPC << EOF
 int main(void) { return 0; }
 EOF
 for flag in $gcc_flags; do
-    if compile_prog "$flag -Werror" "" ; then
+    if compile_prog "-Werror $flag" "" ; then
 	QEMU_CFLAGS="$QEMU_CFLAGS $flag"
     fi
 done