If you are talking about the flags that are passed to each package's configure scripts, then... well... it depends on how they're built. If the package is built using dh_auto_configure (i.e. the dh7 short form), the default set of flags passed can be found in /usr/share/perl5/Debian/Debhelper/Buildsystem/autoconf.pm. Whereas, if it's build using CDBS, then the corresponding flags can be found in /usr/share/cdbs/1/class/autotools-vars.mk.However, these are merely standard flags and it's up to each individual package maintainer to add to or override them.I know, debian don't have to same mechanism as gentoo (debian have very good packages support and package handling utility), but all packages have to have some default EXTRA_FLAGS when was compiling as packages. Because for example mc is compiling as default with vfs support or with X support ..... And I need to know, if is some web site, where can I found this informations. For some packages is it sometime important.
Hope that helps.