Optional dependencies

[rofl0r]

Is your feature request related to a problem? Please describe.
currently in the process of packaging this for my build-from-source distro (so i can run it on my desktop and try out the joystick issue i reported earlier, but this time using a screen on which i can actually read the text).
so far i've had to add libenet and libmpeg2, which weren't in the distro so far.
that however went surprisingly smoothly - both build fine without any additional dependencies.
the next stop now is libserialport, and this one's got a list of about 5 other libraries itself.
i'm pretty confident that i won't need the functionality related to serial ports.
portmidi will come next, if i ever get across the libserialport hurdle. this is also a library nobody's ever needed in the 14 years of my distro's existence.

Describe the solution you'd like
it would be nice if the build could be configured to disable optional functionality. midi, udp networking, mpeg decoding, serial port, is all very unlikely to be needed by someone trying to play games.