Add dolfin/common/types.h and dolfin/common/constants.h as dependencies
for all compiled modules. Forcing petscy4py.i to always be %included
Move population of DOLFIN_PYTHON_DEFINITIONS up one level in the CMake hiearchy,
so we can use DOLFIN_PYTHON_DEFINITIONS when creating dolfin-config.cmake. This in turn
makes it possible to include -DHAS_PETSC4PY for extension module compilations