[Feature Request] Add external build tool support

[MrcSnm]

That is super easy to do and may be included in the future.

This is an example of what need to be done:

The dub.json will need to have

"targetType": "external", //Specifies that buildCommand will be used instead, so, redub won't control cache for that project.
"buildCommand": "make",
"libs": ["lib1", "lib2"] //Will specify that the root project will need those libraries to link

[kassane]

Based on rust-crate cc-rs: https://github.com/kassane/zcc-d/tree/main/samples