[#99] add mingw runner

.gitlab-ci.yml

 variables:
   RUN:
     value: "all"
-    description: "The tests that should run. Possible values: ubuntu, win, all."
+    description: "The tests that should run. Possible values: ubuntu, win-msvc, win-mingw, all."
   GIT_DEPTH: 1
 
 stages:
@@ -34,7 +34,7 @@ build_ubuntu_gcc:
 build_win10_msvc:
   only:
     variables:
-      - $RUN =~ /\bwin.*/i || $RUN =~ /\ball.*/i
+      - $RUN =~ /\bwin-msvc.*/i || $RUN =~ /\ball.*/i
   stage: build
   tags: 
     - win10-msvc-cached
@@ -51,6 +51,23 @@ build_win10_msvc:
     - cmake -DCMAKE_BUILD_TYPE=Debug ..
     - cmake --build .
 
+build_win10_mingw:
+  only:
+    variables:
+      - $RUN =~ /\bwin-mingw.*/i || $RUN =~ /\ball.*/i
+  stage: build
+  tags: 
+    - win10-mingw-cached
+  variables:
+    GIT_SUBMODULE_STRATEGY: recursive
+  timeout: 10m
+  retry: 0
+  script:
+    - mkdir debug
+    - cd debug
+    - cmake -DCMAKE_BUILD_TYPE=Debug ..
+    - cmake --build .
+
 deploy_doc_develop:
   only:
     variables: