bugfix for missing mechanism paths
bugfix for missing mechanism paths

The mechansims/gpu and mechanisms/multicore paths were not being generated
by CMake during configuration, and modcc was silently ignoring that it was
not able to write to the nonexistant target paths.
I have updated CMake to generate the missing gpu and multicore target paths.

halfflat/nestmc-proto,halfflat/arbor,halfflat/nestmc-proto,halfflat/arbor,halfflat/nestmc-proto,eth-cscs/nestmc-proto,eth-cscs/nestmc-proto,halfflat/arbor,halfflat/arbor,halfflat/arbor,eth-cscs/nestmc-proto