Support for macro defined string constants

[azerupi]

I am trying to make bindings for the OpenFX API and they define some constants using macros:

#define kOfxImageEffectPluginApi "OfxImageEffectPluginAPI"
#define kOfxImageEffectPluginApiVersion 1

Bindgen seems to have no trouble with the numeric constant, but it ignores the string constant. Would it be possible to add support for macro defined string constants?

[emilio]

I guess we can try, I'll take a look :)

[emilio]

Patch up in #260, with a bunch of other improvements.

[azerupi]

That was fast! Thank you :)