I think there's an exception that says that OS libraries don't count, since that would mean that you could not write open-source software on any commercial OS.

I think there's an exception that says that OS libraries don't count, since that would mean that you could not write open-source software on any commercial OS. I had a difficult time figuring this out when I released a plugin for the closed-source software LCDC under the GPL. While it's clear that you can't link a GPL library into a non-GPL executable, the reverse case seems to be more muddy. I finally settled to my satisfaction that the spirit of the GPL should not prevent me from open-sourcing a library even if that library only is meaningful when used with a closed-source executable, given that I as the copyright holder can state the intent that the library be linked with proprietary software. It's sort of like the proprietary software is my "OS" and I'm linking to it.