[swift-evolution] [swiftpm] Add proposal for C language support

[Daniel Dunbar]

Happy 2016!

I am working on an initial proposal for adding support for C language targets to the Swift package manager, and am interested in feedback:
  https://github.com/ddunbar/swift-evolution/blob/master/proposals/NNNN-swiftpm-c-language-targets.md

Some TL;DR:
 - The proposal defines a basic convention for pure C language targets (no Swift/C mix and match, but other Swift targets can use the C targets).
 - This is just intended to be the minimal initial feature, there will be a lot of add on work which I expect should be tackled in follow on PRs/proposals.
 - The proposal doesn't try and outline every single nitty detail (e.g., exactly what C++ standard we will compile with). My intention is to pick a sensible default at implementation time and refine incrementally.

Unless there are serious objections, I am hoping to hope to land this proposal soon and start work on the feature shortly after.

Cheers,
 - Daniel