fix deprecated `gcc::Config`

[poga]

build with rustc v1.20.0 will show the following warning:

$ cargo build
    Updating registry `https://github.com/rust-lang/crates.io-index`
   Compiling gcc v0.3.54
   Compiling bitflags v0.1.1
   Compiling libc v0.2.32
   Compiling lua v0.0.11 (file:///Users/poga/projects/spacer-workspace/rust-lua53)
warning: use of deprecated item: gcc::Config has been renamed to gcc::Build
   --> build.rs:136:22
    |
136 |     let mut config = gcc::Config::new();
    |                      ^^^^^^^^^^^^^^^^
    |
    = note: #[warn(deprecated)] on by default

    Finished dev [unoptimized + debuginfo] target(s) in 11.25 secs

gcc::Config has been renamed to gcc::Build.

[jcmoyer]

Thanks.