The commit 2f895225 ("regex: add regexec_buf() that can work on a
non NUL-terminated string", 2016-09-21) changed description of
NO_REGEX build config variable to be more neutral, and actually say
that it is about support for REG_STARTEND.  Change description in
configure.ac to match.

Change also the test message and variable name to match.  The test
just checks that REG_STARTEND is #defined.
Issue-found-by: NRamsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: NJakub Narębski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>