add package portmidi

PortMidi is a cross-platform MIDI library.  Its webpage says:

  "It supports real-time input and output of MIDI data using a
  system-independent interface. PortMidi runs on Windows (using MME),
  Macintosh (using CoreMIDI), and Linux (using ALSA)."

The build system is cmake.  A patch was necessary to build without a
Java Native Interface dependency (only needed for Java bindings and
Java-based utilities).

The pm_test/test.c program is compiled in order to prove that building
works.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>