Switch POSIX/C runtime APIs in preprocessor

[Grissess]

I'd really like to make this a little easier to port to different places; I think the current problem is going to be making sure we can switch on/off (with preprocessor macros) the building of certain operations/sections of code that depend heavily on platform- or runtime-dependent libraries (like libc or libdl).

The only problem with doing so is that I don't have any other targets well in mind to provide an alternative; may I provide a suggestion for a bare-bones system, like a binary for the PDP-11, or a silly little kernel for x86? Switching targets in the C compiler would probably be the easiest way to detect platform-unsafe operations.