Add a hack to disable AUTO_OBJ for 'old style' kernel builds.

Another solution would be to extend the Makefile.sys.inc idea, or a .no_obj
file, to more places but I would rather keep that limited to the top-level
build for now to not impact performance (statting a file in every make call)
or to bring unintended side-effects.

Reported by:	jhb, imp
Sponsored by:	Dell EMC Isilon

share/mk/src.sys.obj.mk

@@ -132,7 +132,8 @@ __objdir:=	${MAKEOBJDIR}
 .if ${.MAKE.LEVEL} == 0 && \
     ${MK_AUTO_OBJ} == "no" && empty(.MAKEOVERRIDES:MMK_AUTO_OBJ) && \
     !defined(WITHOUT_AUTO_OBJ) && !make(showconfig) && !make(print-dir) && \
-    !defined(NO_OBJ)
+    !defined(NO_OBJ) && \
+    empty(RELDIR:Msys/*/compile/*)
 # Find the last existing directory component and check if we can write to it.
 # If the last component is a symlink then recurse on the new path.
 CheckAutoObj= \