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.
It reads a CD's table of contents (TOC) and generates an identifier which
can be used to lookup the CD at MusicBrainz (http://musicbrainz.org).
Additionally, it provides a submission URL for adding the DiscID to
the database.
The interface of this library is new, but the DiscID algorithm and the
operating system dependent CD-ROM/DVD-ROM access code have been ported
from libmusicbrainz version 2.
