Improve portability of build.rs

[Jacherr]

Right now, build.rs uses a very hacky approach to compile the native libvips code by calling gcc directly. This has a number of disadvantages, including poor error handling and portability. It would be ideal to research build.rs and find a more portable way of handling building, as well as having better error handling in the event the native files fail to compile, since right now the only error on such an occurrence is an extremely large and ugly ld error.