Add three missing CMake variables that need to make it to the
LLVMConfig.cmake file that is (I think) used in the stand-alone Clang
build, and causing link errors there w.r.t. curses.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187950 91177308-0d34-0410-b5e6-96231b3b80d8