set flex start conditions from bison?

Is it possible to set the flex start conditions from within bison? I've set a couple of lexer start 
conditions in the flex file and added yy_push_state() to the bison file, which already includes a flex-
generated header. Nonetheless, the compiler still complains that the start condition symbol (named s_element) 
is undeclared. As the start condition symbol is described in the flex file as:

#ifdef YY_HEADER_EXPORT_START_CONDITIONS
#define INITIAL 0
#define s_element 1

#endif

That leads me to believe that some other tweaking is needed to at least define 
YY_HEADER_EXPORT_START_CONDITIONS and therefore declare the start condition symbol.

So, how do we set flex start conditions from within a bison file?


Thanks in advance,
Rui Maciel