Teaching rustpkg how to build C libraries

[catamorphism]

Sub-bug of #5677

As we discussed today ( https://etherpad.mozilla.org/Rustpkg-Servo-meeting ) this can be done in two ways: shell out and hope there's a C compiler in $PATH, or link rustpkg with the libclang driver. The latter is preferable but is a lot of work.

[graydon]

nominating for feature completeness. we know folks will need this.

[catamorphism]

A step towards completing this issue would be to provide a hook that a package script can call to declare a C source file as a dependency ("declared input" for workcache). There would also have to be another hook that a package script could implement to build a given C library from its sources.

[catamorphism]

accepted for feature-complete