Support some method of determining if a symbol is available in dynamic libraries

[ericonr]

Hi!

As it stands, most -sys crates, together with even libc, can't query if an external library contains a certain symbol or not. This makes it so these crates target a specific version of a library, instead of being able to adapt to changes in such libraries (for example libgit2-sys and the issues in rust-lang/libc#1848), unless one is using bindgen or similar.

I am aware this crate doesn't want to carry functionality for generating binaries or dynamic libraries (#46, #250, #458), but, as far as I know, those are the only two ways to make the linker check if all symbols can be resolved, which would in turn allow for "primitive" feature detection.

I'd like to use such a feature to implement rust-lang/libc#2088 (comment). Do you think it's feasible to build it atop cc-rs, or, given this justification, do you think cc-rs could carry it?