On the other hand, there doesn't seem to be any documentation for building from the repository. Since the build process is not exactly straightforward, here is a little recipe.
First you want to clone the repository:
$ git clone git://github.com/MatzeB/libfirm
$ cd libfirm
Then you need to generate some source files from scripts.
$ python scripts/gen_ir.py spec ir/ir
$ python scripts/gen_ir_io.py spec ir/ir
The rest is standard autotools build, except there is no autogen.sh.
$ automake --add-missing
Now configure and make.