Build libpcrecpp.

This is needed for the CrOS crash_reporter. Includes:

* New Android.mk targets for building libpcrecpp (static + shared).

* Manually instantiated headers (pcrecpparg.h, pcre_stringpiece.h).

* Exposing new headers (pcrecpp.h, pcre_scanner.h + aforementioned two)
  via symlinks.

* Small fix in pcrecpp.cc for eliminating unused parameters warning.

These are all orthogonal to the building of the base libpcre target and
so shouldn't affect any users of this library.

Bug: 22486906
Change-Id: Ibdae02fbeb53e8be0f6e6b08e94a4889d0c61cf7
8 files changed