maint: prefer mcel

This causes Gnulib code to also use mcel, which is more consistent.
* bootstrap.conf (avoided_gnulib_modules): Avoid mbuiter
and mbuiterf, since we can now just use mcel.  This avoids
the need to ship and compile mbchar and these modules.
(gnulib_modules): Change mcel to mcel-prefer.

bootstrap.conf

@@ -20,6 +20,8 @@
 avoided_gnulib_modules='
   --avoid=canonicalize-lgpl
   --avoid=dummy
+  --avoid=mbuiter
+  --avoid=mbuiterf
 '
 
 # gnulib modules used by this package.
@@ -163,7 +165,7 @@ gnulib_modules="
   mbschr
   mbslen
   mbswidth
-  mcel
+  mcel-prefer
   memcasecmp
   memchr
   memcmp2