(gl_REGEX): Don't check for off_t, since the code

(gl_REGEX): Don't check for off_t, since the code
no longer needs it.  Instead, check that regoff_t is as least
as wide as ptrdiff_t.

Don't define _REGEX_WIDE_OFFSETS unless using the included regex,
so that our regex.h stays compatible with the installed regex.
This is helpful for installers who configure --without-included-regex.
Problem reported by Emanuele Giaquinta.