Honor --with-mounthelperdir where applicable

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6962

cmd/mount_zfs/Makefile.am

@@ -7,6 +7,8 @@ DEFAULT_INCLUDES += \
 #
 # Ignore the prefix for the mount helper.  It must be installed in /sbin/
 # because this path is hardcoded in the mount(8) for security reasons.
+# However, if needed, the configure option --with-mounthelperdir= can be used
+# to override the default install location.
 #
 sbindir=$(mounthelperdir)
 sbin_PROGRAMS = mount.zfs