pcre: Drop internal libpcre copy
See the commit messages for details. This adds a subproject for the wrapdb copy of libpcre (currently version 8.37), and changes the build to use that instead of the internal libpcre copy. This drops code and makes maintenance easier.
Note that I needed to make some local changes to the wrap patches for libpcre, to enable Unicode support. They are awaiting review in wrapdb here: https://github.com/mesonbuild/pcre/pull/4
Fixes: #962 (closed), #642 (closed)