It took four hours to track the problem down. The GNU make as of 3.80
has an undocumented option called -B which is --always-make which forces
all targets to be built.
The BSD make's -B is a compatibility option which our bsd.port.mk was
specifying.
But since both make's pass their make flags via the same environment
variable, our -B was propogating into the GNU -B. This was causing
the GNU configure subsystem to loop.
I've removed the option. Of course, that might break other things, but
we definitely can't pass it down so if it does break other things we'll
have to fix them case by case.
-Matt