cmake: manipulate CMAKE_MODULE_PATH as a list

Also prefer local modules.

CMakeLists.txt

@@ -7,7 +7,8 @@ set(CMAKE_C_STANDARD 99)
 SET(CMAKE_C_EXTENSIONS OFF)
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/bin")
 add_definitions("-Wall -Wextra -Wno-unused-parameter -D_GNU_SOURCE")
-set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} ${PROJECT_SOURCE_DIR}/CMake)
+list(INSERT CMAKE_MODULE_PATH 0
+  "${CMAKE_CURRENT_SOURCE_DIR}/CMake")
 
 add_subdirectory(swaybg swaybg)