Fix linking problems in static mode.

[pkestene]

Describe your changes here:

A possible fix of issue #531
It feels more appropriate to rename source file for having c++ extension.
Feel free to close this MR if you prefer to keep c extension, and modify the Makefile.am instead.

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General

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• New source/header files are properly added to the Makefiles

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Licence

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[holke]

Thank you for this suggestion. It looks like a good fix for the issue.
Somebody from our team will review your code.

In the meantime, could you please add a file to the doc/ folder stating that you put your contributions under BSD.
You can copy and adapt one of the existing author_* files.

[holke]

Thanks :)