Specifically, this fixes the build of dependent packages (like net/libsoup24)
under Linux, which attempt (and fail) to link in -ldb4 as requested by
${PREFIX}/lib/libproxy.la.
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.
libproxy is a library that provides automatic proxy configuration management.
libproxy offers the following features:
* extremely small core footprint (< 35K)
* no external dependencies within libproxy core (libproxy plugins may
have dependencies)
* only 3 functions in the stable external API
* dynamic adjustment to changing network topology
* a standard way of dealing with proxy settings across all scenarios
* a sublime sense of joy and accomplishment
