Previous revision didn't fix all cases. For example,

testing for variables that are always defined (e.g.,
"make -V CC") would still print a false warning.  Fix
this by only passing a submake the MAKEOBJDIRPREFIX=*
argument if it was present.  As a result, we loose
the check for -DMAKEOBJDIRPREFIX, or an esoteric
"MAKEOBJDIRPREFIX = foo" (with embedded spaces), but
these are unorthodox enough to not care about them.

The make(1) bug mentioned in the previous revision
was just fixed in make/main.c,v 1.109.

Makefile

@@ -86,8 +86,9 @@ TGTS+=	${BITGTS}
 
 PATH=	/sbin:/bin:/usr/sbin:/usr/bin
 MAKEOBJDIRPREFIX?=	/usr/obj
-_MAKEOBJDIRPREFIX!= /usr/bin/env -i PATH=${PATH} ${MAKE} ${.MAKEFLAGS} \
-		-f /dev/null -V MAKEOBJDIRPREFIX dummy
+_MAKEOBJDIRPREFIX!= /usr/bin/env -i PATH=${PATH} ${MAKE} \
+    ${.MAKEFLAGS:MMAKEOBJDIRPREFIX=*} \
+    -f /dev/null -V MAKEOBJDIRPREFIX dummy
 .if !empty(_MAKEOBJDIRPREFIX)
 .error MAKEOBJDIRPREFIX can only be set in environment, not as a global\
 	(in /etc/make.conf) or command-line variable.