I'm working on a commercial closed-source app and I need to use a library under the Cecill-B licence. I read the licence terms here and it desn't say it is mandatory to distribute the source code of the software that uses the library, or to use it only for open-source apps. So I conclude I can use the library for my app, and distribute it in any mean I want (as long as part 5.3.3 of the licence is pretty generic about the use of external modules).
I'm no lawyer, and don't even pretend to play one on the 'net. Take the following with a heavy dose of salt.
I don't see how "firstly, compliance with the principles governing the distribution of Free Software: access to source code, broad rights granted to users" (principles of the license) hint at allowing you to close it up. You should distribute the version you use, in source. Check the exact conditions for distributing changed versions.
But it says about "modules" (essentially, defined as other code linked to the covered code):
5.3.3 DISTRIBUTION OF EXTERNAL MODULES When the Licensee has developed an External Module, the terms and conditions of this Agreement do not apply to said External Module, that may be distributed under a separate license agreement.
I.e., your code is yours to do as you wish.