Add mechanism to detect or set SRC_DIR

The build system will check for the following cases:

 1) CMake build system + source at the same location

 2) CMake build system and source at different location
      a) If SRC_DIR variable is passed, it will use this location
      b) If no SRC_DIR variable is passed, will default to ../cpython-2.7.3
Merge branch 'set-python-source-dir' into next

* set-python-source-dir:
  Add mechanism to detect or set SRC_DIR
  Reference source files using SRC_DIR variable