sys: add runtime feature for bindgen

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

Author: hhirtz

metis-sys/Cargo.toml

@@ -12,4 +12,4 @@ keywords = ["graph", "mesh", "matrix", "partitioning", "ordering"]
 
 
 [build-dependencies]
-bindgen = { version = "0.65", default-features = false }
+bindgen = { version = "0.65", default-features = false, features = ["runtime"] }