[llvm] [llvm-mt] Do not build llvm-mt if not functional (PR #134631)

[Hans Wennborg via llvm-commits]

https://github.com/zmodem approved this pull request.

D'oh, for some reason I didn't see your change to llvm/tools/llvm-mt/CMakeLists.txt, which is the important one. Now it makes more sense :)

https://github.com/llvm/llvm-project/pull/134631