cmCommandLineArgument: Improve error handling and improve `=` support
requested to merge robertmaynard/cmake:cmCommandLineArgument_understands_exact_versus_matching_variables into master
Allows us to provide better error messages when commands such as --targetNoSep
are passed, plus allows us to support --target=<target>
syntax.
Fixes:#22187 (closed)
Edited by Robert Maynard