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.
The Advanced Linux Sound Architecture (ALSA) provides audio and MIDI
functionality to the Linux operating system. ALSA has the following
significant features:
* Efficient support for all types of audio interfaces, from consumer sound
cards to professional multichannel audio interfaces.
* Fully modularized sound drivers.
* SMP and thread-safe design.
* User space library (alsa-lib) to simplify application programming and
provide higher level functionality.
* Support for the older Open Sound System (OSS) API, providing binary
compatibility for most OSS programs.
