Add GNUstep build to Gitlab CI/CD

* .gitlab-ci.yml (test-gnustep): New test target.

.gitlab-ci.yml

@@ -65,3 +65,29 @@ test-filenotify-gio:
     - ./configure --without-makeinfo --with-file-notification=gfile
     - make bootstrap
     - make -C test autorevert-tests filenotify-tests
+
+test-gnustep:
+  stage: test
+  # This tests the GNUstep build process
+  only:
+    changes:
+      - .gitlab-ci.yml
+      - configure.ac
+      - src/nsfns.m
+      - src/nsfont.m
+      - src/nsgui.h
+      - src/nsimage.m
+      - src/nsmenu.m
+      - src/nsselect.m
+      - src/nsterm.h
+      - src/nsterm.m
+      - src/nsxwidget.h
+      - src/nsxwidget.m
+      - src/macfont.h
+      - src/macfont.m
+  script:
+    - DEBIAN_FRONTEND=noninteractive apt install --no-install-recommends -y -qq -o=Dpkg::Use-Pty=0 gnustep-devel
+    - ./autogen.sh autoconf
+    - ./configure --without-makeinfo --with-ns
+    - make bootstrap
+    - make install