Add .gitlab-ci.yml - build and test with meson.

.gitlab-ci.yml

+# This file is a template, and might need editing before it works on your project.
+# use the official gcc image, based on debian
+# can use verions as well, like gcc:5.2
+# see https://hub.docker.com/_/gcc/
+image: gcc
+
+build:
+  stage: build
+  # instead of calling g++ directly you can also use some build toolkit like make
+  # install the necessary build tools when needed
+  before_script:
+    - apt update && apt -y install meson ninja
+  script:
+    - meson build
+  #artifacts:
+  #  paths:
+      - mybinary
+      # depending on your build setup it's most likely a good idea to cache outputs to reduce the build time
+      # cache:
+      #   paths:
+      #     - "*.o"
+
+# run tests using the binary built before
+test:
+  stage: test
+  script:
+    - meson test