for python loading libjackserver and using controlAPI.
drivers are linked to a second instance of libjackserver.so and the
global symbol _jack_get_microseconds showed up uninitialised.

by providing a function pointer to right instance in the engine struct,
we can fix this problem. since _jack_get_microseconds is a in systemdeps,
we need a generic way to get the right function pointer.

git-svn-id: svn+ssh://jackaudio.org/trunk/jack@4421 0c269be4-1314-0410-8aa9-9f06e86f4224