Using vsnprintf or vasprintf in sid

Hi,

I have a need for printf-like printing into a buffer, however I have no 
control over the input and so I don't know the size of the buffer which 
will be required. It looks like vsnprintf (_ISOC99_SOURCE) or vasprintf 
(_GNU_SOURCE) are what I need. Is it ok to use these function in sid 
and, if so, how do I get configure to add the necessary -Dxxxx to the 
sid compilations and (if necessary) specify the library to look in?

Thanks,
Dave