Add support for specifying external libraries by filename

Some libraries have multiple filenames:

set(ZLIB_NAMES z zlib zdll zlib1 zlibd zlibd1)

There is currently no way to tell rustc that #[link(name = "z")] should be satisfied by zlib1.dll.

[metajack]

Brian and I discussed this last week in the context of glfw-rs (the glfw lib is called glfw3 on some distros of Linux and glfw on others). Brian suggested that there be some way to specify a mapping from link names to actual libraries.

cc @bjz

[metajack]

nominating

[pnkfelix]

Not accepted for 1.0, but important nonetheless so P-high.

[steveklabnik]

triage: nothing has changed with link in a while.

[steveklabnik]

Triage: no change here. That said, I would expect that maybe this is something that should be going through Cargo and build scripts, rather than modifying link?

[nagisa]

This seems fairly close to being closed. The need is satisfied by the build scripts, which is the generally accepted way to do complex linking decisions.