cmake: fix variable name

PROJECT_SOURCE_DIR is not yet defined

CMakeLists.txt

@@ -15,7 +15,7 @@ SET(CXX_DISABLE_WERROR True)
 SET(CMAKE_VERBOSE_MAKEFILE True)
 
 # Check if the submodule cmake have been initialized
-IF(NOT EXISTS "${PROJECT_SOURCE_DIR}/cmake/base.cmake")
+IF(NOT EXISTS "${CMAKE_SOURCE_DIR}/cmake/base.cmake")
   MESSAGE(FATAL_ERROR "\nPlease run the following command first:\ngit submodule update --init\n")
 ENDIF()