Attempt to compile with custom llvm on MacOS

cmake/bornagain/modules/SetUpMacOS.cmake

 set(BORNAGAIN_ARCHITECTURE macosx)
 set(BORNAGAIN_PLATFORM macosx)
 
+if(POLICY CMP0025)
+    cmake_policy(SET CMP0025 NEW)
+endif()
 #---------------------------------------------------------------------------------------------------------
 
 if (CMAKE_SYSTEM_NAME MATCHES Darwin)