For the impatient, try this:
> git clone git://github.com/linuxcnc/linuxcnc.git linuxcnc-dev > cd linuxcnc-dev/src > ./autogen.sh > ./configure --with-realtime=uspace > make
That will probably fail! That doesn’t make you a bad person, it just means you should read this whole document to find out how to fix your problems. Especially the section on Satisfying Build Dependencies.
If you are running on a realtime-capable system (such as an install from the LinuxCNC Live/Install Image, see the Realtime section below), one extra build step is needed at this time:
> sudo make setuid
After you’ve successfully built LinuxCNC it’s time to run the tests:
> source ../scripts/rip-environment > runtests
This might fail too! Read this whole document, but especially the section on Setting up the test environment.