Fixed podspec and build errors

Jeff,

Most of your warnings were coming from your previous configuration's declaration of {h,hex,ixx, txx} as _source_ files rather than just _header_ files. You had them in both places but that didn't work because, as the compiler lovingly (and verbosely) let you know, it didn't know how to **compile** those headers, which is true, it can't.

On the other hand, you don't appear to have any actual source files to compile as far as I can see. I'm not sure if your intention is just to use this vendored static library forever or if you want to move to a setup where you're actually compiling the source (recommended), but in either case, you don't have any source files currently. This was a problem because Cocoapods expects there to be *something* to compile… so I appeased it by making a simple dummy C file that would allow it to compile. Since your repo doesn't have that file, you'll need to edit the :git repo in the podspec once you merge this PR so that it pulls from your repo again, not mine.

… now that I think about it, that dummy file was a bad idea because you'll probably get a symbol collision when you attempt to compile it inside an app that defines a main() function. If you remove the main() from the C file I think it should still pass lint. I'll leave that to you to figure out though. If nothing works, let me know and I'll think of another workaround.

And of course, let me know if you have other questions or problems.