make variable for GPL compliant builds

[Daniel Pocock]

I have a couple of projects that include some non-GPL friendly code

However, such code is optional and can be disabled

I would like to give people a way to set some variable that excludes all
the non-GPL stuff, so they can build a re-distributable binary, e.g.

   make GPL_FREE_BINARIES=1 all

Is there any standard name for such a variable, or has this been
contemplated?

Note that this is not just for autotools projects, it would be useful to
use some variable name that ant or cmake projects could recognise too.