Merge branch 'gitlab-ci' into 'master'

Add gitlab ci build description.

See merge request xiph/libao!3

.gitlab-ci.yml

+default:
+  tags:
+    - docker
+  # Image from https://hub.docker.com/_/gcc/ based on Debian
+  image: gcc:9
+
+autoconf:
+  stage: build
+  before_script:
+    - apt-get update &&
+      apt-get install -y libpulse-dev zip
+  script:
+    - ./autogen.sh
+    - ./configure
+    - make
+    - make distcheck