Log message:
Pull in lang/python/extension.mk when buildlinking.

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.

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:
This packages wants python >= 2.5.

Log message:
ntohl etc are in POSIX, no need for _NETBSD_SOURCE

Log message:
Add back a CFLAGS hack for NetBSD overlooked by me in last commit.

Log message:
Make it compile on DragonFly (and probably on some other systems).

Log message:
Import libproxy version 0.2.3.

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