CMake: use CMAKE_CURRENT_SOURCE_DIR

Closes #878

CMakeLists.txt

@@ -28,8 +28,8 @@ string(REPLACE "$(EFFECTIVE_PLATFORM_NAME)" "" PANDA_CFG_INTDIR "${CMAKE_CFG_INT
 
 # Add generic modules to cmake module path,
 # and add Panda3D specific modules to cmake module path
-set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/modules/")
-set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/macros/")
+set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules/")
+set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_SOURCE_DIR}/cmake/macros/")
 
 if(CMAKE_VERSION VERSION_GREATER "3.8")
   # When using the Xcode generator, don't append the platform name to the