Reduce build times on AppVeyor (optimize gyp files)

[wilhelmberg]

@rclark @brianreavis
Another thought on build times on AppVeyor (referencing #19 (comment)):

Probably optimizing the gyp files could be another approach.

At the moment there are a lot of gyp files, e.g. for the formats.
These result in 71 Visual Studio solutions and 71 libraries.

Maybe reducing those could speed up things.
I also think less gyp files would be easier to maintain.

Including a lot of files (sources, headers) is as easy as it gets.

What do you think?
Perhaps @springmeyer could chime in with his knowledge about gyp?

[springmeyer]

combining the gyp files into one might help, but we'd need to test and I've not found time. speaking out loud: I wonder if gyp via a custom python script could be used to combine then on the fly so that they could still stay separate in git for clean organization.

[springmeyer]

going to close this since it feels not super actionable. We can re-open when if rises to importance.