Replaces ${CMAKE_SOURCE_DIR} by ${PROJECT_SOURCE_DIR}

When using a top level CMakeLists.txt ${CMAKE_SOURCE_DIR} will contain
the directory containing the top level CMakeLists.txt and....
${CMAKE_SOURCE_DIR}/libfm-qt will be an non existing directory. It breaks
the build.

${PROJECT_SOURCE_DIR} contains the full path to the root of the project
source directory, i.e. to the nearest directory where CMakeLists.txt
contains the PROJECT() command. That's precisely what we want!

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>