Use gcc::Build given deprecation warning
Use gcc::Build given deprecation warning

Avoids the following warning, as observed with gcc crate 0.3.54:

  warning: use of deprecated item 'gcc::Config': gcc::Config has been renamed to gcc::Build
  --> fasthash-sys/build.rs:52:26
  ...
   = note: #[warn(deprecated)] on by default

flier/rust-fasthash,flier/rust-fasthash