[CMake] diagnosing "could not COPY_FILE" during configure

[Simmons, Aaron]

>> Actually, I did start with a clean tree (rm -rf *).
>>
>If you start with a clean tree again but without --debug-trycompile it works?

If I start with a clean tree without --debug-trycompile it fails (could not COPY_FILE). If I start with a clean tree with --debug-trycompile it will succeed.