Tracking issue for customizing CRT linkage

[alexcrichton]

Tracking issue for rust-lang/rfcs#1721

• - Add the crt-static target-feature
• - Update Cargo to pass through target-feature to build scripts - Expose rustc cfg values to build scripts cargo#3243
• - Implement "lazy link attributes" and implement in a gated fashion in liblibc.

Be aware that "lazy link attributes" (#[link(name = "foo", cfg(foo))]) will never be stable.