Conditional generation of Grammar rules

[Jeganatan, Srividhya]

All,
Is there a way to conditionally generate grammar rules? 
For example, I need to do something like

nonterminal X:
#ifdef FEATURE
 { rule 1 }
#else
 { rule 1; rule 2 }
#endif

I know that ifdefs dont work with the grammar but is there anything else
that can be done?
Or is it possible to make bison to read define variables from a make file?

I want to avoid writing different parsers / recursive parsers to do this.

Thanks for your help.

Srividhya.