What is the proper way for allowing non-gpl plugins/modules on a GPL application?

Question

I develop an open-source CMS, which is licensed under GPLv3, and I want to open the plugin/modules architecture for anyone to contribute their own extensions. However I want to give contributors the freedom to choose whatever license they want for their extensions, and not force them to use the main application's license.

My understanding is that a plain GPL license will force them to release their extensions as GPL code too, however since these are extensions and not core functionality of the application, I'm not clear as to what is the stand of GPL here, or if there is a more appropriate open source license.

Answer 1

There's a chart here summarising various different FOSS licences and the first column comments on proprietary software linking. It doesn't look particularly up to date and doesn't include licence version but it may still be a place to start.

Answer 2

I believe the standard thing to do is to write an exception clause into your license. GCC does this, for example:

As a special exception, you may use this file as part of a free software library without restriction. Specifically, if other files instantiate templates or use macros or inline functions from this file, or you compile this file and link it with other files to produce an executable, this file does not by itself cause the resulting executable to be covered by the GNU General Public License. This exception does not however invalidate any other reasons why the executable file might be covered by the GNU General Public License.