Don't leak -lmingw32 in LIBS

configure.ac

@@ -442,10 +442,12 @@ dnl Check for broken versions of mingw-runtime compatability library
     CPPFLAGS="${CPPFLAGS} -D__USE_MINGW_ANSI_STDIO=1"
 
 dnl Check for the need to include the mingwex lib for mingw32
+    VLC_SAVE_FLAGS
     AC_CHECK_LIB(mingwex,opendir,
         AC_CHECK_LIB(mingw32,opendir,,
             [VLC_ADD_LIBS([libvlccore],[-lmingwex])])
     )
+    VLC_RESTORE_FLAGS
 
 dnl Check for fnative-struct or mms-bitfields support for mingw32
     VLC_SAVE_FLAGS