Use PROJECT_SOURCE_DIR instead of CMAKE_SOURCE_DIR for gitrevision.h
Use PROJECT_SOURCE_DIR instead of CMAKE_SOURCE_DIR for gitrevision.h

Using PROJECT_SOURCE_DIR generates gitrevision.h inside of the cgreen
directory even when used as a submodule. Previously it was generated in
the top level source folder.

Reference:
+ https://cmake.org/cmake/help/v3.0/variable/CMAKE_SOURCE_DIR.html
+ https://cmake.org/cmake/help/v3.0/variable/PROJECT_SOURCE_DIR.html

cgreen-devs/cgreen,cgreen-devs/cgreen,cgreen-devs/cgreen,cgreen-devs/cgreen,cgreen-devs/cgreen