Include RE2 tarball in SVN and future releases.

[Reviewed by Lydia]

As with LLVM and Clang, include an RE2 tarball that will be
unpacked if CHPL_REGEXP=re2. That way, it will not be necessary
to also download RE2 and the Chapel release (or SVN checkout)
will have everything that is needed.



git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@22154 3a8e244f-b0f2-452b-bcba-4c88e055c3ca