Eliminate CMAKE_SOURCE_DIR from CMakeLists.txt

This PR contains a commit that eliminates CMAKE_SOURCE_DIR in include_directories() because include_directories is evaluated relative to the current source directory (i.e. CMAKE_CURRENT_SOURCE_DIR) and replaces the remaining CMAKE_SOURCE_DIR instance with CMAKE_CURRENT_SOURCE_DIR.

Rationale for the change: When Thor is included in another CMake project via add_directories, CMAKE_SOURCE_DIR is the top-level project, not Thor.  This can lead to incorrect path construction.  In the above case, usage of CMAKE_CURRENT_SOURCE_DIR yields the correct paths.

Tested with CMake 2.8.11.2.