[physfs] FTBFS on hurd/i386

[Ryan C. Gordon]

> physfs is unable to compile on hurd, see [1]:
> 
> [ 12%] Building C object CMakeFiles/physfs.dir/platform/unix.o
> /build/buildd/libphysfs-2.0.0/platform/unix.c:26:23: error: sys/mount.h:
> No such file or directory

I think we only use this for the ucred.h path; I'll fix that.

What does the hurd use for detecting mount points? mntent.h? sys/mnttab.h?

> /build/buildd/libphysfs-2.0.0/platform/unix.c:232: error: 'MAXPATHLEN'
> undeclared (first use in this function)

Is this in limits.h?

--ryan.