Improve the GitSharp autotools stuff and move it to its own file,
included in the top-level Makefile.
A system-installed nant is now required to build GitSharp, instead of
using the in-tree copy.
All the files we really need to build GitSharp are now included in the
tarball, including the binary deps. I see weeping packagers...
But "make distcheck" doesn't work, mostly because GitSharp insists on
writing to the source dir during compilation. As distcheck builds with a
read-only source tree, this fails horribly.
