Resolve "MacOS Support"

Closes #86 (closed)

.gitlab-ci.yml

 variables:
   RUN:
     value: "all"
-    description: "The tests that should run. Possible values: ubuntu, win-msvc, win-mingw, all."
-  GIT_DEPTH: 1
+    description: "The tests that should run. Possible values: ubuntu, win-msvc, win-mingw, mac, all."
+  GIT_DEPTH: 15
 
 stages:
   - build
@@ -68,6 +68,25 @@ build_win10_mingw:
     - cmake --no-warn-unused-cli -DCMAKE_EXPORT_COMPILE_COMMANDS:BOOL=TRUE -DCMAKE_BUILD_TYPE:STRING=Debug -DCMAKE_C_COMPILER:FILEPATH=C:\msys64\mingw64\bin\x86_64-w64-mingw32-gcc.exe -DCMAKE_CXX_COMPILER:FILEPATH=C:\msys64\mingw64\bin\x86_64-w64-mingw32-g++.exe .. -G "Unix Makefiles"
     - cmake --build . -j 8
 
+build_mac_clang:
+  only:
+    variables:
+      - $RUN =~ /\bmac.*/i || $RUN =~ /\ball.*/i
+  stage: build
+  tags: 
+    - catalina-clang-cached
+  variables:
+    GIT_SUBMODULE_STRATEGY: recursive
+  timeout: 15m
+  retry: 1
+  script:
+    - mkdir debug
+    - cd debug
+    - export LDFLAGS="-L/usr/local/opt/llvm/lib"
+    - export CPPFLAGS="-I/usr/local/opt/llvm/include"
+    - cmake -DCMAKE_C_COMPILER="/usr/local/opt/llvm/bin/clang" -DCMAKE_CXX_COMPILER="/usr/local/opt/llvm/bin/clang++" -DCMAKE_BUILD_TYPE=Debug ..
+    - cmake --build .
+
 deploy_doc_develop:
   only:
     variables: