configure: Avoid duplicate flags when calling compile_prog

This patch removes redundant shell code and cleans it a little bit.

Shell macro compile_prog takes two arguments:
local_cflags and local ldflags.

$QEMU_CFLAGS is added automatically to the cflags, so there is no need
to pass it as an argument.

It is also unnecessary to pass -Werror twice.

$flag is a compiler warning option, so it should be in local_cflags
instead of local_ldflags.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -1070,7 +1070,7 @@ cat > $TMPC << EOF
 int main(void) { return 0; }
 EOF
 for flag in $gcc_flags; do
-    if compile_prog "-Werror $QEMU_CFLAGS" "-Werror $flag" ; then
+    if compile_prog "$flag -Werror" "" ; then
 	QEMU_CFLAGS="$QEMU_CFLAGS $flag"
     fi
 done