simplify build file

[totaam]

At 2.6kloc, it's just really hard to read and maintain.
At the very least, refactor the almost identical Cython blocks such as this one:

if jpeg_encoder_ENABLED:
       jpeg_pkgconfig = pkgconfig("libturbojpeg")
       add_cython_ext("xpra.codecs.jpeg.encoder",
               ["xpra/codecs/jpeg/encoder.pyx"],
               **jpeg_pkgconfig)
``

[totaam]

See also 3b40fc0

This will do for now, more will be done as part of #3592.