Support mcel API for apps that prefer it.
This mirrors the recent changes to mbsstr.
* lib/mbscasestr.c [GNULIB_MCEL_PREFER]: Include mcel.h not mbuiter.h.
(mbchar_t, mb_equal) [GNULIB_MCEL_PREFER]: New type and function,
to make it easier to use common code.
(knuth_morris_pratt_multibyte): Don't assume mbchar_t's alignment
is at least that of size_t.
(knuth_morris_pratt_multibyte, mbscasestr) [GNULIB_MCEL_PREFER]:
Use mcel API.
* modules/mbscasestr (Depends-on): Add alignasof.