meson: work around broken system PCRE2 dependency in macOS

meson: work around broken system PCRE2 dependency in macOS

macOS provides a PCRE2 library in base that is not usable and not
configured properly, as it installs a pkgconf module that
points to a non-existent pcre2.h header in /usr/local/include.

Detect that case and if the feature is enabled, try to fallback
to a wrapped subproject through an anonymous dependency, aborting
with an error if that is not possible.

Change the feature to "auto" and print a warning and disable it
if a broken dependency was detected, but to keep consistency
with the cmake build system used on Windows, add a special rule
to re-enable the pcre2 feature by default there.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Suggested-by: Eli Schwartz <eschwartz@gentoo.org>
Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>