More and more Debian source packages offer extra functionnality, besides building their regular binary packages. For instance, the source packages for gcc and binutils allow you to build cross-compilers, and emdebian offers tools for (cross-)building stripped-down versions of some source packages.
Debconf-src is a (mockup) proposal for standardizing access
to such extra functionnality. It consists in a dpkg-buildpackage
wrapper, called debconf-buildpackage,
which allows its user to access alternative build configurations
through Debconf questions.
It should be noted that debconf-buildpackage configures the source package
only through environment variables, and will usually use its own
Debconf database rather than polluting the system's one.
Effort has been made to respect the debconf namespace nonetheless.
A preliminary debconf-src package is available in the repository,
with manual pages documenting debconf-buildpackage and the interface.
Feel free to have a look and send me
your comments.