[Ur] Patches to compile Ur/Web with clang

[Adam Chlipala]

austin seipp wrote:
> I've been learning Ur/Web a bit recently, and on my OS X machine I
> encountered some failures when attempting to build the compiler using
> clang for the C code.
>
> So I made up a small patch and fixed the build issues.
>    

Thanks!  I've pushed your changes to the main Ur/Web repo, with a few 
minor variations.  Let me know if my version doesn't work for you.