question about package source configuration

[Patrick Hsieh <pahud@pahud.net>]

Hello,
After I apt-get source mysql-gpl-client
then cd mysql-gpl-*/
I can dpkg-buildpackage -b -uc to build the binary.
But how can I modify the ./configure parameters?

I checked Makefile.in and found $(EXTRA_CONF_ARGS). 
Looks like I can put my args in that variable. But 
where is that varible defined?

Thanks.