gitlab: add variable for make command name

To facilitate future jobs that will use FreeBSD
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

.gitlab-ci.yml

+variables:
+  MAKE: make
+
 .job_template: &job_definition
   script:
     - mkdir build
     - cd build
     - ../autogen.sh $CONFIGURE_OPTS || (cat config.log && exit 1)
-    - make -j $(getconf _NPROCESSORS_ONLN)
+    - $MAKE -j $(getconf _NPROCESSORS_ONLN)
 
 # We could run every arch on every versions, but it is a little
 # overkill. Instead we split jobs evenly across 9, 10 and sid