Add a static pkg-config file for re2.

This allows packages using re2 to easily discover it and query how to compile
and link against it. For more details see:
http://people.freedesktop.org/~dbn/pkg-config-guide.html

Change-Id: I6c7a7a17947954c163f653b97980ac49f3bd95a4
Reviewed-on: https://code-review.googlesource.com/2520
Reviewed-by: Paul Wankadia <junyer@google.com>