Hello hackers,

Justus tried to build Guix on his Hurd machine and he found out that
even though we disable (guix build syscalls) from building when
sys/mount.h is not present, it still tries to build it.

As I found out, (guix utils) module uses the syscalls module so that's
why it still tried to build it. That's why I followed a different approach.
I delayed the evaluation of ptr and proc on mount, umount, swapon, etc.
and it builds now.

WDYT? If you agree with the change I will push it to wip-hurd and/or
master.

Manolis