[feature] add gitlab CI pipeline

Simple gitlab-ci config with a single step, running the full list of
configure, make, make install and make check.

No tags, no split build and test for now, as artifacts are a bit tricky
to get right.

This config should grow in the future to ensure that we run all the test
on all the platforms we want.

.gitlab-ci.yml

+stages:
+  - build
+
+make:generic:
+  stage: build
+  script:
+    - ./autogen.sh
+    - mkdir build
+    - ./configure --prefix=`pwd`/build
+    - make
+    - make install
+    - make check
+  artifacts:
+    when: on_failure
+    paths:
+      - tests/*.log