vfs:glusterfs_fuse: build only if we have setmntent()

vfs:glusterfs_fuse: build only if we have setmntent()

FreeBSD and other platforms that don't have setmntent() and friends can
not compile this module. This patch lets changes the build to only
compile this module if the setmntent() function is found.

This is the a follow-up fix to the actual fix for bug #13972.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13972

Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
Autobuild-User(master): Amitay Isaacs <amitay@samba.org>
Autobuild-Date(master): Thu Aug  1 09:49:04 UTC 2019 on sn-devel-184