Link to clang at runtime

[hhirtz]

this makes bindgen load libclang during the execution of build instead of its compilation.

it also has the advantage that older versions of clang are supported. see the following links for statements:

• https://github.com/rust-lang/rust-bindgen/issues/1694
• https://github.com/Janekdererste/metis-rs#why-this-fork