Add makefile to compile with cosmopolitan libc

[skeeto]

Thanks! I squashed the fixup commit before merging.

The fsum issue is interesting, and I'm surprised cosmopolitan casually carves out this name as reserved under external linkage. Since it's been given such importance, I'm also surprised after looking at the source it's a naive sum (despite the recursion) rather than a more precise sum like Kahan.