try_compile: run each check in its own directory instead of CMakeTmp
--debug-trycompile, it should not reuse the same CMakeTmp dir, but a different one for each call.
This would make it much easier to troubleshoot issues in failed
try_compile calls without having to manually add a
message(FATAL_ERROR) in the project just after the relevant
try_compile call, to ensure it doesn't get overridden by another one.