Prefer to use CMAKE_CURRENT_SOURCE_DIR instead of CMAKE_SOURCE_DIR for includes

[abdes]

This has the benefit of working better and more reliably when the project is included in another master project. We can guarantee that the includes still come from the sub-project as expected.

# List of directories specifying a search path for CMake modules to be loaded by
# the the include() or find_package() commands before checking the default
# modules that come with CMake.
list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake)