better control over make.conf?

[l0kit0 at exactas.org]

hi,
ive seen a feature from gentoo that i really liked, and i wanted to know if
fbsd have something like it, i mean the /uses/ variables in make.conf, where
one can specify better control over make,
for example (from
http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?part=2&chap=2):

"Let us take a look at a specific example: the kde keyword. If you do not
have
this keyword in your USE variable, all packages that have optional KDE
support will be compiled without KDE support. All packages that have an
optional KDE dependency will be installed without installing the KDE
libraries (as dependency). If you have defined the kde keyword, then those
packages will be compiled with KDE support, and the KDE libraries will be
installed as dependency."

salute