Support environment variable in "cc-flags" & "cc-link-flags"

[tonyfettes]

Feature Request

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Summary

Support usage of environment variables in "cc-flags" & "cc-link-flags" field.

Motivation

For native backend programs, I want my problem to be able to compile on linux & macOS, which sometimes may link to different system libraries, or the same library but located at different place.

However, a proper multi-platform support is somewhat complex and relies on us to maintain a list of supported platform. By using environment variables, we can let users to handle the complexity of the platforms they will be using.

Detailed Description

I would be able to write:

{
  "link": {
    "native": {
      "cc": "gcc",
      "cc-flags": "${CFLAGS}"
      "cc-link-flags": "${LDFLAGS}"
    }
  }
}

And moon will read the two environment variables from its environment, and interpolate them into the flags string before calling moonc.

Related Issues

Checklist

• I have added all necessary details to make it easy for the maintainers to understand my request.