Parallelise building GParted in GitLab CI jobs (!6)

Reduce the time taken by the GitLab Continuous Integration jobs by
parallelising make to use all available CPUs in the Docker CI image
when it is building GParted code.  This includes 'make diskcheck'
because that also does a second build of the GParted code in a separate
subdirectory.

Closes !6 - Reduce the time taken by the GitLab CI jobs

.gitlab-ci.yml

@@ -24,18 +24,22 @@ stages:
   stage: build
   script:
     - ./autogen.sh
-    - make
+    - nproc=`grep -c '^processor' /proc/cpuinfo` || nproc=1
+    - echo nproc=$nproc
+    - make -j $nproc
     - make install
 
 .test_stage_template: &test_stage_definition
   stage: test
   script:
     - ./autogen.sh
-    - make
+    - nproc=`grep -c '^processor' /proc/cpuinfo` || nproc=1
+    - echo nproc=$nproc
+    - make -j $nproc
     # Exclude specific unit test which fails without /dev/disk in Docker images.
     - export GTEST_FILTER='-BlockSpecialTest.NamedBlockSpecialObjectBySymlinkMatches'
     - make check
-    - make distcheck
+    - make -j $nproc distcheck
 
 # Check GParted can be built and installed on CentOS and Ubuntu.
 centos_build: