Fix depends, remove cairomm/libtool,-ltdl & agg. Mapnik uses its own
internal agg fork now. Fix WANTLIB while here, and ensure dependency on
png 1.6, otherwise build breaks horribly.
Force the dependency on scons 2.3.0, newer mapnik switched from CXXFLAGS
to CPPDEFINES, and with previous scons this produces broken build flags like
-DHAVE_JPEG-DMAPNIK_USE_PROJ4-DHAVE_PNG-DHAVE_TIFF-DBIGINT-DOPENBSD-DMAPNIK_THREADSAFE
in turn producing broken builds.
Add a warning about the 'dangerous' regress tests.
47e2e40500