gitlab-ci: Add a build description.

Describe builds for the gitlab continuous integration
service runners. This does a trial build under both
autotools and cmake, so we get some coverage for changes
on that hosting platform.

After the same script in the vorbis and ogg projects.

Signed-off-by: Mark Harris <mark.hsj@gmail.com>

.gitlab-ci.yml

+default:
+  tags:
+    - docker
+  # Image from https://hub.docker.com/_/gcc/ based on Debian
+  image: gcc
+
+autoconf:
+  stage: build
+  before_script:
+    - apt-get update &&
+      apt-get install -y zip doxygen
+  script:
+    - ./autogen.sh
+    - ./configure
+    - make
+    - make distcheck
+  cache:
+    paths:
+      - "src/*.o"
+      - "src/.libs/*.o"
+      - "silk/*.o"
+      - "silk/.libs/*.o"
+      - "celt/*.o"
+      - "celt/.libs/*.o"
+
+cmake:
+  stage: build
+  before_script:
+    - apt-get update &&
+      apt-get install -y cmake ninja-build
+  script:
+    - mkdir build
+    - cmake -S . -B build -G "Ninja" -DCMAKE_BUILD_TYPE=Release
+    - cmake --build build