Never used it myself but there is Tilcon.  They say Linux support is coming.

> We are also interested in developing our user interfaces in a way that they 
> will run on QNX/Photon and Windows. The preferred programming language would 
> be C++.
> 
> So far I was only able to find projects that are discontinued on the QNX side 
> some years ago.
> 
> I was able to get gtk compiling on QNX 6.3.2 using the pkgsrc project on this 
> site. But gtk will relay on an X11 server. That may work, but XPhoton, the X11
>  server on Photon, is not available with an actual X.org releases. I do not 
> know if anybody will move the old xfree86 server to an up to date X.org based 
> server.
> 
> An alternative would be to port gtk+ (gdk) to Photon. Gtk+ does already
> run natively on X11 (that was the start), win32, quartz and directfb.
> All of these ports are part of the gtk+ source. As it is stated that Photon
> was designed with Xt in mind the port may be not that hard. But that is my 
> estimation and I do not know Xt nor the Photon internals. Has anybody tried 
> this way?
> 
>