w32: Fix build with older gcc (unresolved symbol)

The cross i586-mingw32msvc-gcc 4.4.4 from Debian Squeeze does not support
__sync_val_compare_and_swap by default.

Using -march=i686 fixes that and should also result in better code.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

configure

@@ -562,6 +562,11 @@ if test "$mingw32" = "yes" ; then
   QEMU_CFLAGS="-DWIN32_LEAN_AND_MEAN -DWINVER=0x501 $QEMU_CFLAGS"
   # enable C99/POSIX format strings (needs mingw32-runtime 3.15 or later)
   QEMU_CFLAGS="-D__USE_MINGW_ANSI_STDIO=1 $QEMU_CFLAGS"
+  if test "$cpu" = "i386"; then
+    # We need something better than i386 for __sync_val_compare_and_swap
+    # and can expect that QEMU will only run on i686 or later.
+    QEMU_CFLAGS="-march=i686 $QEMU_CFLAGS"
+  fi
   LIBS="-lwinmm -lws2_32 -liphlpapi $LIBS"
 cat > $TMPC << EOF
 int main(void) { return 0; }