Makefile & configure: reword inaccurate comment about PCRE

Makefile & configure: reword inaccurate comment about PCRE

Reword an outdated & inaccurate comment which suggests that only
git-grep can use PCRE.

This comment was added back when PCRE support was initially added in
commit 63e7e9d8b6 ("git-grep: Learn PCRE", 2011-05-09), and was true
at the time.

It hasn't been telling the full truth since git-log learned to use
PCRE with --grep in commit 727b6fc3ed ("log --grep: accept
--basic-regexp and --perl-regexp", 2012-10-03), and more importantly
is likely to get more inaccurate over time as more use is made of PCRE
in other areas.

Reword it to be more future-proof, and to more clearly explain that
this enables user-initiated runtime behavior.

Copy/pasting this so much in configure.ac is lame, these Makefile-like
flags aren't even used by autoconf, just the corresponding
--with[out]-* options. But copy/pasting the comments that make sense
for the Makefile to configure.ac where they make less sense is the
pattern everything else follows in that file. I'm not going to war
against that as part of this change, just following the existing
pattern.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>