Build script overrides cannot use target.<cfg>.<links> form

[glandium]

Problem

As of writing the only way to set build script overrides is to use target.<triple>.<links>. Unfortunately, when you want to set those overrides for all targets at once, or for some subsets, the only option right now is to set them for each and every triple. This doesn't really scale.

Cc: @thomcc

Proposed Solution

No response

Notes

No response

[eerii]

At gstreamer-rs, we are trying to support linking to a big static library with all dependencies inside, and we need to override the individual dependency library names. We use system-deps to link the libraries on the command line, which handles this automatically. However, in Windows, variables need to have dllimport applied to them as opposed to functions which export symbols with and without __imp_. The only way to do this right now seems to be using the #[link] attribute, but this forces us to set the name of the library there. It would be very useful to use the links overrides to rename this library from dependent crates, however, the current design is very limiting. Not only it needs to be repeated for all targets, but it also doesn't allow for configuration based on features or cfg flags. Commenting on this issue to show how it would be useful for our use case.