Ich zitier mal die FSF: "How can I allow linking of proprietary modules with my GPL-covered library under a controlled interface only? (#LinkingOverControlledInterface)
Add this text to the license notice of each file in the package, at the end of the text that says the file is distributed under the GNU GPL:

Linking ABC statically or dynamically with other modules is making a combined work based on ABC. Thus, the terms and conditions of the GNU General Public License cover the whole combination.

In addition, as a special exception, the copyright holders of ABC give you permission to combine ABC program with free software programs or libraries that are released under the GNU LGPL and with independent modules that communicate with ABC solely through the ABCDEF interface. You may copy and distribute such a system following the terms of the GNU GPL for ABC and the licenses of the other code concerned, provided that you include the source code of that other code when and as the GNU GPL requires distribution of source code.

Note that people who make modified versions of ABC are not obligated to grant this special exception for their modified versions; it is their choice whether to do so. The GNU General Public License gives permission to release a modified version without this exception; this exception also makes it possible to release a modified version which carries forward this exception.

Only the copyright holders for the program can legally authorize this exception. If you wrote the whole program yourself, then assuming your employer or school does not claim the copyright, you are the copyright holder—so you can authorize the exception. But if you want to use parts of other GPL-covered programs by other authors in your code, you cannot authorize the exception for them. You have to get the approval of the copyright holders of those programs."

Kurzum das Mischen ist nicht erlaubt, das Verweisen aber schon. Nicht nur ich interpretiere den Passus so.