CI: Using multiple stages/jobs

* .gitlab-ci.yml: here.

.gitlab-ci.yml

-debian-stable-gcc:
+.pre-build:
+  stage: build
   only:
     - branches
   image: registry.lrde.epita.fr/nolimips-sid
-  script:
+  before_script:
     - ./bootstrap
     - ./configure
-    - make -k
-    - make -k check
-    - make -k distcheck
+
+make:
+  extends: .pre-build
+  script:
+    - make
+  
+check:
+  extends: .pre-build
+  script:
+    - make check
+
+distcheck:
+  extends: .pre-build
+  script:
+    - make distcheck