Simplify build.rs of fitsio-sys

[RReverser]

While trying to add alternative Windows build mode, I couldn't help but notice that build.rs has a lot of duplicate code that could be better expressed through cfg branching.

I extracted it as such, so now each build step is defined only in one place.

[simonrw]

Hey thanks for this PR. I am glad someone wanted to tackle simplifying the build script.

We have had some unrelated issues with the CI that are now preventing the CI from passing. Before I review this properly, would you mind rebasing onto main and force pushing, so we can rely on the CI to say if the changes are functionally correct?

[cjordan]

I'm unable to robustly test this right now, but the code looks good 👍

[simonrw]

I think we have enough coverage in the tests where we use all combinations of the with-bindgen and cfitsio-src features to show that this implementation works. I also think the code restructuring is nice, leaving just the unique parts of each code path in the main function (which is feature dependent) and moving the bindings generation into its own function.

You mention that you are working on a Windows build for this project. It would be great to increase our Windows support generally, as we don't have a lot of coverage in that area. Please keep us posted on your progress there, and feel free to reach out with any questions!