Support linking libpg_query dynamically

[lubo]

Currently, the code below always forces static linking:

libpg_query-sys/build.rs

Line 10 in 3ce5062

println!("cargo:rustc-link-lib=static=pg_query");

[autarch]

I'm not really using this crate these days, but if you want to make a PR to enable both static & dynamic linking I'd be happy to review it.