buildworld: Define SYSROOT to WORLDTMP.

This is to allow downstream Makefiles to know for sure they are building
against a sysroot rather than only depending on ${DESTDIR} or other
assumptions.

This also exports it into buildenv.

MFC after:	2 weeks
Sponsored by:	Dell EMC Isilon

Makefile.inc1

@@ -563,7 +563,8 @@ KTMAKE=		TOOLS_PREFIX=${WORLDTMP} MAKEOBJDIRPREFIX=${WORLDTMP} \
 # world stage
 WMAKEENV=	${CROSSENV} \
 		INSTALL="sh ${.CURDIR}/tools/install.sh" \
-		PATH=${TMPPATH}
+		PATH=${TMPPATH} \
+		SYSROOT=${WORLDTMP}
 
 # make hierarchy
 HMAKE=		PATH=${TMPPATH} ${MAKE} LOCAL_MTREE=${LOCAL_MTREE:Q}
@@ -722,7 +723,7 @@ IMAKE_MTREE=	MTREE_CMD="mtree ${MTREEFLAGS}"
 .endif
 
 # kernel stage
-KMAKEENV=	${WMAKEENV}
+KMAKEENV=	${WMAKEENV:NSYSROOT=*}
 KMAKE=		${KMAKEENV} ${MAKE} ${.MAKEFLAGS} ${KERNEL_FLAGS} KERNEL=${INSTKERNNAME}
 
 #

Makefile.libcompat

@@ -111,6 +111,7 @@ LIBCOMPATCXXFLAGS+=	-isystem ${LIBCOMPATTMP}/usr/include/c++/v1 -std=c++11 \
 LIBCOMPATWMAKEENV+= MAKEOBJDIRPREFIX=${LIBCOMPAT_OBJTREE} \
 		INSTALL="sh ${.CURDIR}/tools/install.sh" \
 		PATH=${TMPPATH} \
+		SYSROOT=${LIBCOMPATTMP} \
 		LIBDIR=/usr/lib${libcompat} \
 		SHLIBDIR=/usr/lib${libcompat} \
 		DTRACE="${LIB$COMPATDTRACE:U${DTRACE}}"