Compile embedded git with USE_LIBPCRE

I use a commit hook that does `git grep -P`, i.e. uses Perl regexps which is a compile time dependency. This works everywhere but Atom's git support, where it fails because that git is built without it.

It seems that just about everyone else compiles with pcre. Short list of git versions we use where this works:

* Linux and Homebrew distributions
* Github Desktop on Mac and Windows
* Recent Apple versions (Apple Git-98). The system (xcode) git used to not have PCRE a while back (when it was an ancient 1.8.something) but it does as of High Sierra.
* Official git-scm.org package for Mac

Having an embedded git, while convenient, should follow mainstream git as closely as possible in my opinion. And PCRE in turn is probably the "mainstream RE" these days and not some esoteric option.

I've taken to telling people who use Atom to simply remove the dugite git-core, forcing it to use system, but this seems sub-optimal if you guys are relying on the behaviors of a fixed version. Eliminating the `grep -P` is non-trivial.

[Re-filed here from https://github.com/desktop/dugite/issues/160 as requested by @shiftkey ]