Ok, doing a little preliminary work, I can't find any non-free licenses in the current release per their own LICENSE.txt and gpl-3.0-exceptions.txt files. Now there could be a incompatible mix of FOSS licenses but I could use a an expert here...

Looking at the licenses that licensecheck recognizes I get this:
$ licensecount .
   9682 *No copyright* UNKNOWN
    741 *No copyright* GENERATED FILE
    489 GPL (v3 or later)
    218 UNKNOWN
     69 LGPL (v2.1 or later) (with incorrect FSF address)
     18 zlib/libpng
     12 GPL (v2 or later)
      3 LGPL (with incorrect FSF address)
      2 GENERATED FILE
      1 BSD (2 clause)


Richard