CMake: Use PROJECT_SOURCE_DIR to improve using mosquitto as subdirectory

[Hallot]

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 [email protected]

• Have you signed the Eclipse Contributor Agreement, using the same email address as you used in your commits?
• Do each of your commits have a "Signed-off-by" line, with the correct email address? Use "git commit -s" to generate this line for you.
• If you are contributing a bugfix, is your work based off the fixes branch?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run make test with your changes locally?

[ralight]

Thank you, I hadn't considered this way of including projects as subdirectories and how it would affect the build.