Build libpcrecpp.

This is needed for the CrOS crash_reporter. Includes:

* New 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 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