Makefile: make USE_LIBPCRE=YesPlease mean v2, not v1

Makefile: make USE_LIBPCRE=YesPlease mean v2, not v1

Change the USE_LIBPCRE flag from being an alias for USE_LIBPCRE1 to
being an alias for USE_LIBPCRE2.

When support for v2 was added in my 94da9193a6 ("grep: add support for
PCRE v2", 2017-06-01) the existing USE_LIBPCRE flag was left as
meaning v1, with a note that this would likely change in a future
release. That optional support for v2 first made it into Git version
2.14.0.

The PCRE v2 support has been shown to be stable, and the upstream PCRE
project is highly encouraging downstream users to move to v2, so it
makes sense to give packagers of Git who haven't heard the news about
PCRE v2 a further nudge to move to v2.

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