Add clang-cl configurations

appveyor.yml

@@ -15,12 +15,20 @@ image:
 environment:
   matrix:
     - GSL_CXX_STANDARD: 14
+      USE_COMPILER: MSVC
+    - GSL_CXX_STANDARD: 14
+      USE_COMPILER: MSVC
     - GSL_CXX_STANDARD: 17
+      USE_COMPILER: clang-cl
+    - GSL_CXX_STANDARD: 17
+      USE_COMPILER: clang-cl
 
 matrix:
     exclude:
     - image: Visual Studio 2015
       GSL_CXX_STANDARD: 17
+    - image: Visual Studio 2015
+      USE_COMPILER: clang-cl
 
 cache:
   - C:\cmake-3.8.0-win32-x86
@@ -47,7 +55,12 @@ before_build:
           $env:generator="$env:generator Win64"
       }
       echo generator="$env:generator"
+      if ("$env:USE_COMPILER=" -match "clang_cl") {
+          cmake .. -G "$env:generator" -T LLVM -DGSL_CXX_STANDARD="$env:GSL_CXX_STANDARD"
+      }
+      else {
           cmake .. -G "$env:generator" -DGSL_CXX_STANDARD="$env:GSL_CXX_STANDARD"
+      }
 
 build_script:
   - cmake --build . --config %CONFIGURATION% -- /m /v:minimal