在clang的travis上使用不同的libc++版本

before_install:
  - export CXX="clang++"
  - export CC="clang"
  - sudo apt-get install -y libc++-dev libc++abi-dev

script:
  - clang++ -std=c++11 -stdlib=libc++ main.cpp -o main
  - ./main

jobs:
  include:
    - env: LIBCXX_VERSION=8
      before_install:
        - export CXX="clang++"
        - export CC="clang"
        - sudo apt-get install -y libc++-8-dev libc++abi-8-dev
      script:
        - clang++ -std=c++11 -stdlib=libc++ main.cpp -o main
        - ./main
    - env: LIBCXX_VERSION=9
      before_install:
        - export CXX="clang++"
        - export CC="clang"
        - sudo apt-get install -y libc++-9-dev libc++abi-9-dev
      script:
        - clang++ -std=c++11 -stdlib=libc++ main.cpp -o main
        - ./main