We put occasional tarballs of development snapshots in the development download area.
To check out a copy of the source:
git clone git://github.com/jcupitt/libvips.git
You'll need to rebuild the configure system after downloading:
cd libvips ./bootstrap.sh
Then you can configure ; make ; make install as usual.
./configure make make install
If you want to make changes and merge them, make an account on github, fork vips, edit as you please, and push commits upstream as you make them. gitref.org is a very handy getting-started guide to git.