Did you upgrade DragonFly on the 25th? If you are on DragonFly-master and you upgraded during select hours on the 25th of February, you may have been bit by a makefile error.  The fix, as listed in that link, is simple:
cp /usr/src/share/mk/sys.mk /usr/share/mk
If you are not on -master or you did not upgrade in that timeframe: never mind.