gstreamer: drop manual CFLAGS

These were cargo-culted from MacPorts. The build system passes different
flags.
gstreamer: set env var instead of appending
gst-plugins-good: set env var instead of appending
enchant: make python an optional dependency.
bottle_version: support fontforge scheme.
hexchat: make python an optional dependency.
pyqwt: make python a hard dependency.
pyqwt: set PYTHONPATH in test.
vte: make python a hard dependency.
vte: add dependency on pygobject.
vte3: make python a hard dependency.