sys.mk: Improve ports compatibility hack guardHEADmain

Back in 9ca543438039 and include of bsd.cpu.mk in sys.mk was added as a
hack for ports Makefiles.  This works fine in general (for values of
fine that include not being able to use MK_ variables in bsd.cpu.mk),
but has surprising effects on MacOS with a case insensitive filesystems.
There we build things in paths like share/docs/IPv6 where
../../Mk/bsd.port.mk matches share/mk/bsd.port.mk.

Fix this issue by checking for ../../Mk/Uses which seems highly unlikely
to match something in share/mk.

Reviewed by:	imp, emaste
Differential Revision:	https://reviews.freebsd.org/D49232

0 files changed, 0 insertions, 0 deletions