Spell out the exports in docs for macOS build

Tell the users, either source `.graphscope.env`
or with this to support openmp

```bash
export CC=/opt/homebrew/opt/llvm/bin/clang
export CXX=/opt/homebrew/opt/llvm/bin/clang++
```

Then I tried manually trigger the build process, with `mkdir build && cd build && cmake ..`

it reports:

_Originally posted by @yecol in https://github.com/alibaba/GraphScope/discussions/2619#discussioncomment-6086610_