[CMake] Use variables rather than ":" delimiters

This is a more idiomatic CMake.

Differential Revision: https://reviews.llvm.org/D37644

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335703 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed