* remember #defined names, so that C.stdout can refer
  to the real name (on OS X) __stdoutp.
* better handling of #defined constant expressions
* allow n, err = C.strtol("asdf", 0, 123) to get errno as os.Error
* write all output files to current directory
* don't require gcc output if there was no input

Fixes #533.
Fixes #709.
Fixes #756.

R=r
CC=dho, golang-dev, iant
https://golang.org/cl/1734047