Use the installed world's idea of OSRELDATE rather than the kernel.

This was the initial intent anyway, and it became clear that it is
really necessary to treat it this way, as many people happen to run
with kernel newer than the installed world.

Submitted by:	imp, ru
Approved by:	re (scottl)

Makefile.inc1

@@ -120,7 +120,8 @@ SUPFLAGS+=	-h ${SUPHOST}
 
 MAKEOBJDIRPREFIX?=	/usr/obj
 .if !defined(OSRELDATE)
-OSRELDATE!=	sysctl -n kern.osreldate
+OSRELDATE!=	awk '/^\#define[[:space:]]*__FreeBSD_version/ { print $$3 }' \
+		/usr/include/osreldate.h
 .endif
 TARGET_ARCH?=	${MACHINE_ARCH}
 .if ${TARGET_ARCH} == ${MACHINE_ARCH}