doxygen fails to build in xenial on amd64

[Matthias Klose]

doxygen fails to build in xenial on amd64, see
https://launchpad.net/ubuntu/+archive/test-rebuild-20160614/+build/9929902

cd /«PKGBUILDDIR»/build/doc && /usr/bin/python3 translator.py
Traceback (most recent call last):
  File "translator.py", line 1993, in <module>
    major, minor, patch = (int(e.strip('+')) for e in platform.python_version_tuple())
  File "translator.py", line 1993, in <genexpr>
    major, minor, patch = (int(e.strip('+')) for e in platform.python_version_tuple())
ValueError: invalid literal for int() with base 10: '2rc1'
doc/CMakeFiles/run_doxygen.dir/build.make:118: recipe for target 'doc/language.doc' failed
make[4]: *** [doc/language.doc] Error 1
make[4]: Leaving directory '/«PKGBUILDDIR»/build'
CMakeFiles/Makefile2:504: recipe for target 'doc/CMakeFiles/run_doxygen.dir/all' failed
make[3]: *** [doc/CMakeFiles/run_doxygen.dir/all] Error 2
make[3]: Leaving directory '/«PKGBUILDDIR»/build'
CMakeFiles/Makefile2:446: recipe for target 'doc/CMakeFiles/docs.dir/rule' failed
make[2]: *** [doc/CMakeFiles/docs.dir/rule] Error 2
make[2]: Leaving directory '/«PKGBUILDDIR»/build'
Makefile:286: recipe for target 'docs' failed
make[1]: *** [docs] Error 2
make[1]: Leaving directory '/«PKGBUILDDIR»/build'
debian/rules:55: recipe for target 'build-stamp' failed
make: *** [build-stamp] Error 2
dpkg-buildpackage: error: debian/rules build gave error exit status 2