Log message:
Simply and speed up buildlink3.mk files and processing.
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.

Log message:
Include pyversion.mk include the protected part of the buildlink3.mk
files, not over and over again.

Log message:
Remove Python 2.1 support.

Log message:
Add DESTDIR support.

Log message:
Update PYTHON_VERSIONS_COMPATIBLE
- assume that Python 2.4 and 2.5 are compatible and allow checking for
fallout.
- remove PYTHON_VERSIONS_COMPATIBLE that are obsoleted by the 2.3+
default. Modify the others to deal with the removals.

Log message:
Fix warning reported by "pkglint".

Log message:
Add buildlink3 support file.

Log message:
Reset maintainer -- shell@ is inactive.

Log message:
allow any Python >=21

Log message:
Consistently use NetBSD.org address for shell, shellhung.org does not exist.