CMake: use bundled PCRE if system PCRE doesn't jit

Summary: PCRE comes with its own CMakeLists.txt, fortunately, so all we have to
do is run it.

Caution: I have very little idea what I'm doing when it comes to CMake
-- there's a lot of hidden magic going on -- but I think this is some
approximation of correct.

Reviewed By: @ptarjan

Differential Revision: D1523103