Currently, all command line option validation occurs after all configure checks, since the configure phase affects what command line options are allowed.
However, this means if you misspell a command line option, you may not get an obvious error; instead you may get a confusing configure failure.

Instead, we should parse all command line options first, to check for obvious problems (invalid command line options under any configuration). Then do configure checks, and finally validate the command line options based on the configuration.