cmake: add pcre2 support

cmake: add pcre2 support

Fix one of the TODOs listed in the CMakeLists.txt by adding support
for building with pcre2.

As pcre2 doesn't provide cmake find module, we find it with pkgconf.
This patch also works with vcpkg on Windows, with pkgconf and pcre2
installed.

Pkgconf and pcre2 is detected automatically just like curl, expat
and iconv. The output of CMake indicates whether pcre2 is found.

Signed-off-by: Yuyi Wang <Strawberry_Str@hotmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>