Remove libselinux.so.1 dependency of the AppImage

[probono]

Dear Subsurface developers,

the binaries inside the Subsurface AppImage require libselinux.so.1
which typically is not available on Arch Linux (e.g., Antergos) in the
default installation. Hence it would be good if it could be disabled
during compilation, or a mock (dummy) .so could be bundled inside the
AppImage, satisfying the dependency.

https://github.com/probonopd/AppImages/issues/83#issuecomment-282566072

Thanks,
Simon