Due to popular demant into each port which might be inserted into dependency
list by bsd.port.mk insert anti foot-shooting device, which prevents
infinite fork loop when the user defines corresponding USE_XXX in global
make.conf, command line or environment.
Similar devices should probably be inserted into ports that might be inserted
into dependency list by others bsd.foo.mk files (bsd.ruby.mk, bsd.python.mk
and so on.)