We can't completely avoid it, since gl's headers use these defines for
their header guards, and we really do need to stop the system GL
headers from doing anything.
Fixes#14
I was thinking at one point that part of the build was going to
require not including the #defines from the generated code, but would
want these prototypes. It turns out that's not the case (and if it
is, I'll just wrap the #defines in an ifdef).