mbschr, mbsrchr: support GNULIB_MCEL_PREFER Support mcel API for apps that prefer it. * lib/mbschr.c, lib/mbsrchr.c: Include stdlib.h, for MB_CUR_MAX. [GNULIB_MCEL_PREFER]: Include mcel.h instead of mbuiterf.h. (mbschr, mbsrchr) [GNULIB_MCEL_PREFER]: Use mcel API.