message: Add multiple Linux jobs (gcc, gcc-8, clang, mingw-w64)
diff: -version: 2.0
  -jobs:
  +version: 2.1
  +
  +commands:
  build:
  - docker:
  - - image: outpostuniverse/nas2d-circleci:1.1
  steps:
  - checkout
  - run: git submodule update --init
  - run: make --keep-going nas2d
  - run: make --keep-going
  +
  +jobs:
  + build-linux:
  + docker:
  + - image: outpostuniverse/nas2d:1.4
  + steps:
  + - build
  + build-linux-gcc8:
  + docker:
  + - image: outpostuniverse/nas2d-gcc8:1.0
  + steps:
  + - build
  + build-linux-clang:
  + docker:
  + - image: outpostuniverse/nas2d-clang:1.0
  + steps:
  + - build
  + build-linux-mingw:
  + docker:
  + - image: outpostuniverse/nas2d-mingw:1.3
  + steps:
  + - build
  +
  +workflows:
  + build:
  + jobs:
  + - build-linux
  + - build-linux-gcc8
  + - build-linux-clang
  + - build-linux-mingw
  