CMake: Use PROJECT_SOURCE_DIR to improve using mosquitto as subdirectory

When mosquitto is included as subdirectory, `CMAKE_SOURCE_DIR` does not refer to the mosquitto top level CMake file, but to the whole project top level CMake.
Use `PROJECT_SOURCE_DIR` instead to refer to the right CMake in both contextes.

Signed-off-by: Pierre Hallot <hallotpierre@gmail.com>