If building with LLVM_ENABLE_DOXYGEN and using CMake then
abort while configuring if doxygen could not be found. This
is desirable because if the build is going to fail then it should
fail as early as possible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207404 91177308-0d34-0410-b5e6-96231b3b80d8