Comment 2 for bug 1802345

> This is a bigger issue when building with VMs in snapcraft 3, as there's no way to export C_INCLUDE_PATH.

You can use the `build-environment` key, refer https://github.com/snapcore/snapcraft/pull/2322 for more info.

For legacy Snapcraft syntax you have to override the build step and build the part manually, or [write a local plugin](https://forum.snapcraft.io/t/writing-local-plugins/5125) to replace/complement [the `python` plugin](https://github.com/snapcore/snapcraft/blob/master/snapcraft/plugins/python.py).