Enable binding of all C functional macros

[Slabity]

So unfortunately, the cexpr crate (and as such, bindgen), does not support C functional macros (jethrogb/rust-cexpr#3), so we can't directly create bindings for them.

We can however, use a neat little hack to get around this, which is to define a constant variable that takes the value of the macro. I've put this workaround into the build script. All we need to do is put each functional macro name into the list of names.