Bug#649708: Please Build-Depend on ghostscript, not gs-common.
magnus currently Build-Depends on gs-common, which is a transitional package.
The next upload of ghostscript (currently) plans to drop both gs-common and
the ghostscript "Provides: gs-common". As soon as that upload happens, it
will make magnus FTBFS.
Please replace the Build-Depends on gs-common to a Build-Depends on ghostscript.