This fixes issue https://github.com/larskanis/fxruby/issues/20

it's better to use runOnUiThread in souch a case.

This should ensure, that the event is definitely cleared.

These functions should be thread safe.

Switch off polling-based (addChore) processing of threads

Add event-based interruption of GVL-released functions.

This allows to safely execute GUI code from other threads.

This works for sockets only for now.

This is the only function that is currently required to acquire the GVL.
Other FXRbCall functions could be wrapped equally.

This is just enough for loadPixel/savePixel, but if other methods
release the GVL, potentially all other callback functions could
be called. These are now yet wrapped by gvl-aware wrappers.

This also fixes some build system checks, that should be for the target system.

without check.