cmake: CompilerChecks: Correct standard warnings

If the -std options don't work for the compiler, issue a
warning, not an error.  The existing error was not using
a valid message type.

This doesn't alter the behaviour of cmake other than the message text.

--no-rebase

/cc @jburel 

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Testing: on MacOS 10.8, you will get a warning message when running cmake.

Run `cmake /path/to/bioformats.git` from a temporary directory.