grep: change the internal PCRE macro names to be PCRE1

Change the internal USE_LIBPCRE define, & build options flag to use a naming convention ending in PCRE1, without changing the long-standing USE_LIBPCRE Makefile flag which enables this code. This is for preparation for libpcre2 support where having things like USE_LIBPCRE and USE_LIBPCRE2 in any more places than we absolutely need to for backwards compatibility with old Makefile arguments would be confusing. In some ways it would be better to change everything that now uses USE_LIBPCRE to use USE_LIBPCRE1, and to make specifying USE_LIBPCRE (or --with-pcre) an error. This would impose a one-time burden on packagers of git to s/USE_LIBPCRE/USE_LIBPCRE1/ in their build scripts. However I'd like to leave the door open to making USE_LIBPCRE=YesPlease eventually mean USE_LIBPCRE2=YesPlease, i.e. once PCRE v2 is ubiquitous enough that it makes sense to make it the default. This code and the USE_LIBPCRE Makefile argument was added in commit 63e7e9d8b6 ("git-grep: Learn PCRE", 2011-05-09). At the time there was no indication that the PCRE project would release an entirely new & incompatible API around 3 years later. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>