configure.ac: improve description of NO_REGEX test

configure.ac: improve description of NO_REGEX test

The commit 2f8952250a ("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: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Jakub Narębski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>