Unbreak the MAKEOBJDIRPREFIX variable check by avoiding bugs in make(1).

When make flags are passed to make in the environment, the string is
chopped up in an (argc,argv) vector. This happens in brk_string() and
the chopped up string is stored in static buffer. When this includes
something like "-V BINMAKE", then a pointer into the static buffer is
put on the variables list for evaluation later. However, brk_string()
is used for more than just chopping up the MAKEFLAGS env. variable, so
it's very likely that the static buffer is clobbered. In fact, this is
exactly what happens.
The result is that _MAKEOBJDIRPREFIX gets assigned whatever garbage
the child make happens to emit, causing the test to fail. Like this:

pluto2% cd /usr/src
pluto2% make -V BINMAKE
"/q/6.x/src/Makefile", line 94: MAKEOBJDIRPREFIX can only be set in environment, not as a global (in /etc/make.conf) or command-line variable.
pluto2% make -dv -V BINMAKE | & grep _MAKEOBJDIRPREFIX
Global:_MAKEOBJDIRPREFIX = }

The fix is to not use MAKEFLAGS for this, but simple pass the flags
as arguments. Ideally make(1) should be fixed but that's beyond the
scope of my attention span.

This fixes release.

Makefile

@@ -86,8 +86,7 @@ TGTS+=	${BITGTS}
 
 PATH=	/sbin:/bin:/usr/sbin:/usr/bin
 MAKEOBJDIRPREFIX?=	/usr/obj
-_MAKEOBJDIRPREFIX!= /usr/bin/env -i \
-		PATH=${PATH} MAKEFLAGS="${.MAKEFLAGS}" ${MAKE} \
+_MAKEOBJDIRPREFIX!= /usr/bin/env -i PATH=${PATH} ${MAKE} ${.MAKEFLAGS} \
 		-f /dev/null -V MAKEOBJDIRPREFIX dummy
 .if !empty(_MAKEOBJDIRPREFIX)
 .error MAKEOBJDIRPREFIX can only be set in environment, not as a global\