swift234

Initial integration of libdispatch into build-script

Extend build-script, build-script-impl, and update-checkout
to include libdispatch.  For now, libdispatch is not
built by default (user must enable via command line
argument).

Integration of testing is functional, but should be improved
in a later pull request.  The basic autotools based test
harness does not give the nice high-level progress output
as the rest of the test suite.

A related pull request to libdispatch (#34) has some fixes
to the autotools build that are needed to enable the test
target to succeed when run in an external directory.