Fix FTBFS on systems without CLOCK_MONOTONIC_RAW

Initialise midi_clockid to the invalid value -1, thus fixing a FTBFS on
systems where CLOCK_MONOTONIC_RAW is not defined.
Fix pixmap installation path

A typo prevented the pixmaps to be installed correctly, either resulting
in a make install error or pixmaps installed to /pixmaps.

This patch fixes the problem.