Cchar sign should be compiler dependent

[vtjnash]

I'm not sure how to detect this, but the C-prefix types are most useful if they actually correspond to the C type they are supposed to be

[JeffBezanson]

I'm not sure it matters. You get the same bit pattern whether you convert to Int8 or Uint8, and the calling convention is surely the same as well.

[vtjnash]

llvm/clang does not consider them the same (remember #978 ?) and has a slightly different calling convention for them on x86

[JeffBezanson]

Ah yes, there is that.