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.
Remember .include "foo.mk" is looked for (first) in the directory that
contains the makefile being processed (like in C), so remove all the
${.PARSEDIR} and ../ sequences that just cause grief.
package is returned rather than querying the source package. First, this
is more correct, and second, it greatly speeds up pkgsrc, especially
when many packages are already installed.
