message: Update Linux documentation
diff: @@ -205,7 +205,9 @@ Your Linux distribution might default to `gcc` instead of `clang`, if that's the
  ```
  $ mkdir build-release
  $ cd build-release
  -$ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=../dist-linux ..
  +# Or use a specific version of clang, for instance /usr/bin/clang-5.0
  +$ CC=/usr/bin/clang CXX=/usr/bin/clang++ \
  + cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=../dist-linux ..
  ```
  And then invoke `ninja`:
  