Replace build script with linkage attributes

[complexspaces]

This PR follows up ff1a6dd with a slight improvement.

Instead of needing a build script to be compiled and ran just to link against system-provided dynamic libraries, it is possible to use the link attribute instead. This has an identical end result to the previous solution since the linker flags are generated just the same, but without the maintenance and build-time downsides of a build script.

[DoumanAsh]

Thanks
This was the first thing I tried when I started removing winapi but I remember having difficulties with using just linkage attributes (that's why it was released as beta.1 rather than full version)
I need to re-check it again to make sure linkage attributes work

[complexspaces]

FWIW, I tested this on Windows 10 and cargo test worked just fine. I commented out the build script and incrementally applied the linkage attributes per extern block, watching the number of unknown symbol errors go down until it successfully compiled, linked, and ran.

Obviously feel free to do your own testing, this is just what appeared to work for me.

[DoumanAsh]

Works without issue, thank you.
I will publish stable 5.0.0