Add a gitlab-ci job for the unix Makefile.

Ensure test coverage for the non-autotools makefile. This is
much faster than the autotools build, but doesn't currently
support tests or a `dist` target.

The syntax of the travis-ci.org build description file makes
it tedious to add alternate build types. It's much easier with
gitlab, so it's nice to have testing outside of jenkins.

.gitlab-ci.yml

 # Image from https://hub.docker.com/_/gcc/ based on Debian
 image: gcc
 
-build:
+autotools:
   stage: build
   before_script:
     - apt-get update &&
@@ -14,3 +14,14 @@ build:
     - make distcheck
   tags:
     - docker
+
+makefile:
+  stage: build
+  before_script:
+    - apt-get update &&
+      apt-get install -y libopus-dev libogg-dev libssl-dev
+  script:
+    - make -C unix
+    - make -C unix check
+  tags:
+    - docker