This disables compilation of this module on GNU/Hurd.
Reported by Manolis Ragkousis <manolis837@gmail.com>.
* m4/guix.m4 (GUIX_CHECK_LIBC_MOUNT): New variable.
* configure.ac: Use it. Define 'BUILD_SYSCALLS_MODULE' conditional.
* Makefile.am (MODULES, EXTRA_DIST): Make 'guix/build/syscalls.scm'
conditional on BUILD_SYSCALLS_MODULE.