FreeBSD and HURD support

It seems that the existing preprocessor variable VCMI_LINUX is only used to switch XDG support on and off. But linux is only the kernel and no description for the userland (which you already realized because of the special casing you do for android) and in addition XDG is not limited to linux. So a better fitting name for that preprocessor variable would be VCMI_XDG which would be switched on on platforms with XDG support. Some linux based platforms are part of those. The BSDs and GNU/HURD based systems are others.

Building on top of that it seems that vcmi builds with only minor modifications on kFreeBSD and HURD. This pull request implements those changes.