CheckIPOSupported: Tolerate backslashes in output of failed checks

Avoid passing the output of our try_compile through a macro argument. Macro invocations re-parse their arguments and so should not be given arbitrary content that may contain backslashes and such.

Instead pass a simple message. This is also more readable in the case that the macro generates a message itself. After the macro returns, report the real output directly to the variable named by our caller. Also record the output in a log file as other checks do on failure.

While at it, simplify checks for whether the caller passed OUTPUT or RESULT.

Fixes: #18244 (closed)