Fix native-xtools build

Fixes		https://github.com/freebsd/poudriere/issues/894
Fixes:		d0c737e18 ("Makefile: Fix MAKEOBJDIRPREFIX command-line")
X-MFC-With:	d0c737e18

Makefile

@@ -218,7 +218,7 @@ _MAKEOBJDIRPREFIX!= /usr/bin/env -i PATH=${PATH} ${MAKE} MK_AUTO_OBJ=no \
     ${.MAKEFLAGS:MMAKEOBJDIRPREFIX=*} __MAKE_CONF=${__MAKE_CONF} \
     SRCCONF=${SRCCONF} SRC_ENV_CONF= \
     -f /dev/null -V MAKEOBJDIRPREFIX dummy
-.if !empty(_MAKEOBJDIRPREFIX) || !empty(.MAKEOVERRIDES:MMAKEOBJDIRPREFIX)
+.if !empty(_MAKEOBJDIRPREFIX)
 .error MAKEOBJDIRPREFIX can only be set in environment or src-env.conf(5),\
     not as a global (in make.conf(5) or src.conf(5)) or command-line variable.
 .endif

Makefile.inc1

@@ -2739,7 +2739,6 @@ SUBDIR=	${SUBDIR_OVERRIDE}
 NXBMAKEARGS+= \
 	OBJTOP=${NXBOBJTOP:Q} \
 	OBJROOT=${NXBOBJROOT:Q} \
-	MAKEOBJDIRPREFIX= \
 	-DNO_SHARED \
 	-DNO_CPU_CFLAGS \
 	-DNO_PIC \
@@ -2763,6 +2762,9 @@ NXBMAKEARGS+= \
 	MK_WERROR=no \
 	MK_ZFS=no
 
+NXBMAKEENV+= \
+	MAKEOBJDIRPREFIX=
+
 .if make(native-xtools*) && \
     (!defined(NXB_TARGET) || !defined(NXB_TARGET_ARCH))
 .error Missing NXB_TARGET / NXB_TARGET_ARCH