[doxygen] Use correct variable names for external variable configuration and make EXTRA_SEARCH_MAPPINGS a "dumb" variable.

I do not think the massaging that I was doing for EXTRA_SEARCH_MAPPINGS was
truly necessary.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189522 91177308-0d34-0410-b5e6-96231b3b80d8