Many developers run with userland != to kernel. While this isn't

supported, it usually works for months at a time.  Allow these people
to override the OSRELDATE of their installed world when things don't
match and the exact OSRELDATE matters and is different than the
kernel.  Now that Makefile.inc1 depends more and more about which date
you have to optimize the pieces it builds, it may be necessary to
pessimize things if its guesses are wrong.

If OSRELDATE is already set, we won't fork the sysctl to find out what
the kernel's date is.

Developers on IRC suggested that they run mismatches all the time as
well.

Reviewed by: obrien

Makefile.inc1

@@ -119,7 +119,9 @@ SUPFLAGS+=	-h ${SUPHOST}
 .endif
 
 MAKEOBJDIRPREFIX?=	/usr/obj
+.if !defined(OSRELDATE)
 OSRELDATE!=	sysctl -n kern.osreldate
+.endif
 TARGET_ARCH?=	${MACHINE_ARCH}
 .if ${TARGET_ARCH} == ${MACHINE_ARCH}
 TARGET?=	${MACHINE}