CMakeLists: Clean up

Moves functions out of the main CMakeLists file into module files that
can just be included whenever necessary. This also uses the CMake
provided variables for enforcing compiler requirements.

src/CMakeLists.txt

@@ -1,4 +1,5 @@
 include_directories(.)
+include(CreateDirectoryGroups)
 
 set(SRCS
     backend_x64/block_of_code.cpp