new 'make installworld' warnings

[Dan Mack]

I buildworld / installworld a few times per week.   I don't think I've 
ever seen these messages before:

make[1]: "/usr/obj/usr/src/compiler-metadata.mk" line 1: Using cached 
compiler metadata from build at cow.example.com on Sun May  7 
09:13:52 CDT 2017
make[3]: "/usr/obj/usr/src/compiler-metadata.mk" line 1: Using cached 
compiler metadata from build at cow.example.com on Sun May  7 
09:13:52 CDT 2017

Possibly due to a significant enough deley between the run of buildworld 
and installworld?  Build and install were otherwise successful.

This happened when transitioning between 317440-stable -> 317906-stable

Dan