Add Project( DD4hep ) call so cmake policies are properly set

Fixes issue that CMAKE_CXX_COMPILER_ID was not AppleClang on MAC

CMakeLists.txt

 cmake_minimum_required(VERSION 3.3 FATAL_ERROR)
+
+PROJECT( DD4hep )
+
 set ( CMAKE_MODULE_PATH      ${CMAKE_MODULE_PATH} ${CMAKE_SOURCE_DIR}/cmake ) 
 #
 include ( DD4hepBuild )