add YAML file for GitLab CI integrated build process

.gitlab-ci.yml

+stages:
+   - build
+
+.build_template:
+   stage: build
+   script:
+      - make realclean
+      - make all
+   only:
+      refs:
+         - master
+         - merge_requests
+      variables:
+         - $CI_PROJECT_URL == 'https://vcgit.hhi.fraunhofer.de/jvet/VVCSoftware_VTM'
+
+.build_template_linux:
+   extends: .build_template
+   script:
+      - make realclean
+      - make all
+      - make realclean
+      - make linuxbuild enable-tracing=true
+
+build_macos:
+   extends: .build_template
+   tags:
+      - macos
+
+build_ubuntu1604:
+   extends: .build_template_linux
+   tags:
+      - ubuntu1604
+
+build_ubuntu1804:
+   extends: .build_template_linux
+   tags:
+      - ubuntu1804
+
+build_ubuntu1804-gcc8:
+   extends: .build_template_linux
+   script:
+      - make realclean
+      - make all toolset=gcc-8
+      - make realclean
+      - make linuxbuild enable-tracing=true toolset=gcc-8
+   tags:
+      - ubuntu1804-gcc8
+
+build_vc1900:
+   extends: .build_template
+   tags:
+      - vc1900
+
+build_vc191x:
+   extends: .build_template
+   tags:
+      - vc191x
+
+build_vc192x:
+   extends: .build_template
+   tags:
+      - vc192x
+