Fix clang not used as compiler in CI

Author: cschreib

.github/workflows/cmake.yml

@@ -13,8 +13,8 @@ jobs:
       fail-fast: false
       matrix:
         platform:
-        - { name: Ubuntu GCC,   os: ubuntu-latest,  compiler: g++,     arch: "64", cmakepp: "",        flags: "-DCMAKE_CXX_FLAGS=--coverage"}
-        - { name: Ubuntu Clang, os: ubuntu-latest,  compiler: clang++, arch: "64", cmakepp: "",        flags: ""}
+        - { name: Ubuntu GCC,   os: ubuntu-latest,  compiler: g++,     arch: "64", cmakepp: "",        flags: "-DCMAKE_CXX_COMPILER=g++ -DCMAKE_CXX_FLAGS=--coverage"}
+        - { name: Ubuntu Clang, os: ubuntu-latest,  compiler: clang++, arch: "64", cmakepp: "",        flags: "-DCMAKE_CXX_COMPILER=clang++ -DCMAKE_CXX_FLAGS=-stdlib=libc++"}
         - { name: Windows 32,   os: windows-latest, compiler: vs2019,  arch: "32", cmakepp: "",        flags: "-A Win32"}
         - { name: Windows 64,   os: windows-latest, compiler: vs2019,  arch: "64", cmakepp: "",        flags: "-A x64"}
         - { name: MacOS,        os: macos-latest,   compiler: clang++, arch: "64", cmakepp: "",        flags: ""}
@@ -32,10 +32,6 @@ jobs:
       with:
         submodules: 'recursive'
 
-    - name: Setup Linux compiler
-      if: runner.os == 'Linux'
-      run: export CXX=${{matrix.platform.compiler}}
-
     - name: Setup Emscripten cache
       if: matrix.platform.compiler == 'em++'
       id: cache-system-libraries