The clutter build failed because GLX_FRONT_LEFT_EXT and
GLX_BIND_TO_TEXTURE_RGBA_EXT were not defined. (I had MesaLib-6.4.2nb3
installed.)
So add BUILDLINK_API_DEPENDS.MesaLib+= MesaLib>=7.0
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.
