Update .gitlab-ci.yml file : add clang and gcc build

.gitlab-ci.yml

+image: debian:latest
+
+.common_build:
+  before_script:
+    - apt update && apt -y install make autoconf qtbase5-dev qtchooser qt5-qmake qtbase5-dev-tools libavutil-dev libavcodec-dev libavformat-dev libmpv-dev cmake clang clang-format gcc g++ libomp-dev
+  script:
+    - mkdir build && cd build
+    - CC=${c_compiler} CXX=${cxx_compiler} cmake .. && make
+
+
+build-clang:
+  stage: build
+  extends: .common_build
+  variables:
+    c_compiler: clang
+    cxx_compiler: clang++
+
+build-gcc:
+  stage: build
+  extends: .common_build
+  variables:
+    c_compiler: gcc
+    cxx_compiler: g++