how to pass #define from .ebuild into compiler?

http://forums.xamarin.com/discussion/45804/specify-define-on-command-line-for-xbuild
The /p: option is for setting a project variable. The "Configuration" and "Platform" variables are for defining the build configuration and target platform, which are common project concepts (they show up in the IDE as things like "Debug|iPhone", where "Debug" is the configuration and "iPhone" is the platform). They aren't #defines. The way you add defines is to have a configuration in your project file that defines those values and then specify that configuration at build time.

https://bugzilla.xamarin.com/show_bug.cgi?id=21521