exception if PETSC_DIR is not defined

[Marco Morandini]

This line

params['build']['lib_dirs'] += [os.environ["PETSC_DIR"] + "/lib"]

in python/dolfin/jit/pybind11jit.py raises an exception if PETSC_DIR is not defined. However, PETSC_DIR is required to be defined only on Macs; Linux boxes work fine without it.

[Prof Garth Wells]

This line looks really fishy - we should be able to get the lib path via pkg-config (from petsc.pc).