build: really disable fusermount if you say so

There is no logic in configure.ac that provides a $disable_fusermount
variable. So, use the $enable_fusermount variable instead.

Follow-up-for: http://review.gluster.org/4773
Change-Id: I81cdbd0045409d0036438d542ca6dc1934f784e4
BUG: 948205
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/4803
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Reviewed-by: Sachidananda Urs <sacchi@gmail.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>