[CMake] Don't use CMAKE_SOURCE_DIR during install

Referring to an installed file using `CMAKE_SOURCE_DIR`
only works when UMF is built and installed standalone,
but not when it's included by other projects.

Only `LICENSE.TXT` and `licensing/third-party-programs.txt` still had
these issue, while other files already used `PROJECT_SOURCE_DIR`.

Author: ProGTX

CMakeLists.txt

@@ -599,10 +599,10 @@ endif()
 # --------------------------------------------------------------------------- #
 # Configure make install/uninstall and packages
 # --------------------------------------------------------------------------- #
-install(FILES ${CMAKE_SOURCE_DIR}/LICENSE.TXT
+install(FILES ${PROJECT_SOURCE_DIR}/LICENSE.TXT
         DESTINATION "${CMAKE_INSTALL_DATAROOTDIR}/doc/${PROJECT_NAME}/")
 install(
-    FILES ${CMAKE_SOURCE_DIR}/licensing/third-party-programs.txt
+    FILES ${PROJECT_SOURCE_DIR}/licensing/third-party-programs.txt
     DESTINATION "${CMAKE_INSTALL_DATAROOTDIR}/doc/${PROJECT_NAME}/licensing/")
 
 install(DIRECTORY examples DESTINATION "${CMAKE_INSTALL_DOCDIR}")