Add ffc calls to demo cmake files

[Martin Sandve Alnæs]

The makefiles of demos should run ffc to produce foo.h from foo.ufl. Then we don't need the generate-form-files script except for the few generated checked in files.

[Martin Sandve Alnæs]

We could

1) add a line at the top of each .ufl file in the demos:

#ffc -l dolfin <arguments>

2) let the build system in dolfin run "ffc Foo.ufl" automatically

3) let ffc look for #ffc and use the arguments "-l dolfin <arguments>"

[Anders Logg (Chalmers)]

Removing milestone: 1.7 (automated comment)

[Martin Sandve Alnæs]

For the record I attempted this here but gave up:

https://bitbucket.org/fenics-project/dolfin/branch/martinal/topic-run-ffc-through-cmake