print "usage" for unknown arguments

[umläute]

after installing xcbuild (via homebrew)I was kind of lost: whatever I tried would result in an unknown argument error:

$ xcbuild -h
error: unknown argument -h
$ xcbuild --help
error: unknown argument --help
$ man xcbuild
No manual entry for xbuild
$

only after searching the sources(sic!), i discovered that in order to get help i must specify -help (with a single dash).
Now I understand that single-dash arguments are somewhat standard with macOS. Unfortunately I'm coming from a GNU world (with single-dash short options and double-dash long options).

I think it would be very nice if:

• any unknown argument would (in addition to triggering the unknown argument %s message) be redirected to -usage.
• include xcbuild -help in the usage examples (probably at the very end).

[Stephane Sezer]

Agreed. I don't have time to deal with this right now, but if somebody ends up submitting a PR I'll gladly accept it.