You may find life simpler if you use debian tools to build a custom deb package (which you can install manually)
apt-get source dolfin
cd dolfin-*
dpkg-buildpackage
cd ..
sudo dpkg -i *dolfin*.deb
Install any missing required build dependencies that might be identified by dpkg-buildpackage.
The dolfin package is the last of the FEniCS components. If you also need to build the others from source then follow the same procedure one by one (fiat, dijitso, ufl, ffc).
If you need to patch the source, consider using quilt, which can create patch files located in debian/patches. Use dch to update debian/changelog for your custom build.