Fix CI: use libc++ on linux with clang

Workarounds actions/runner-images#8659

.github/workflows/on_pull_request.yml

@@ -106,14 +106,16 @@ jobs:
  - uses: actions/checkout@v2
  - name: Install Dependencies
  run: |
- sudo apt install -y clang lld
+ sudo apt install -y clang lld "libc++"
  - name: Configure
  run: |
  set -x
  mkdir Clang
  cd Clang
  export CC=$(which clang)
  export CXX=$(which clang++)
+ export CXXFLAGS=$(-stdlib=libc++)
+ export LDFLAGS=$(-stdlib=libc++)
  cmake -DCMAKE_CXX_STANDARD=20 ..
  - name: Build
  run: |