Eliminate CMAKE_SOURCE_DIR from CMakeLists.txt.

Specifically, this commit 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: 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.