gitlab-ci: Add a make check step.

Invoke `make check` at the end of the default build, since we
don't do `make distcheck` for all the jobs. At the moment this
does nothing, but it should give better coverage if any test
harnesses are hooked up to the target.

.gitlab-ci.yml

@@ -13,6 +13,7 @@ default:
     - ./autogen.sh
     - ./configure ${CONFIG_FLAGS}
     - make
+    - make check
 
 autoconf:
   extends: .autoconf