Support user passing a custom CMAKE_MODULE_PATH (for BLAS / LAPACK).

- Previously we set CMAKE_MODULE_PATH explicitly rather than appending
  to it when adding our local cmake directory.
- This meant the user could not easily pass their own directory which
  would also be used.  This would be useful if they had their own
  custom FindBLAS / FindLAPACK scripts that they wanted to overload
  the defaults shipped with CMake.

Change-Id: I0c1fda1b6fd81eab68755aa036f64af3efbb226d