This changes the buildlink3.mk files to use an include guard for the
recursive include. The use of BUILDLINK_DEPTH, BUILDLINK_DEPENDS,
BUILDLINK_PACKAGES and BUILDLINK_ORDER is handled by a single new
variable BUILDLINK_TREE. Each buildlink3.mk file adds a pair of
enter/exit marker, which can be used to reconstruct the tree and
to determine first level includes. Avoiding := for large variables
(BUILDLINK_ORDER) speeds up parse time as += has linear complexity.
The include guard reduces system time by avoiding reading files over and
over again. For complex packages this reduces both %user and %sys time to
half of the former time.
For now, DragonFly and FreeBSD use the libc version, it is not reentrant,
but thread-safe. NetBSD 3.0+ and Darwin 8.0+ use libresolv from base
(the BIND9 resolver), all other fall back to net/bind9. Feel free to add
your favorite platform if it has a thread-safe resolver in base.
Modify mail/libspf-alf, mail/milter-greylist, mail/spamdyke and
net/nocol accordingly. Testing on !DragonFly and feedback from tron@
