fix OSX architecture detection for i386

(cherry picked from commit 0fea6563)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

configure.ac

@@ -135,7 +135,7 @@ case "${host_os}" in
     dnl Force gcc "-arch" flag
     ARCH_flag=""
     case "${host}" in
-      i386*)
+      i?86*)
         ARCH_flag="-arch i386"
       ;;
       ppc64*)