Support build without pkg_config.

The existing build.rs script requires pkg_config to be present. While this is fine for POSIX systems, on Windows it makes it tricky to build without running the build inside msys (which is impractical for certain projects). This change adjusts the build.rs script to only use pkg_config if on non Windows system.

On Windows, it is now the library consumer's responsibility to make slirp available on the library search path.