There was a great cross-platform environment called Visix Galaxy in the 1990's. You could compile your C++ source into native apps that ran on Windows NT as well as Solaris and HP-UX. It had a fantastic GUI builder which is similar to what is available today.
It's possibly still around: http://www.ambiencia.com/overviewgalaxytd.php . Expensive as fuck, though; if I understood the pricing page correctly, it's $1.5K for a single-developer no-support license of the cheapest version. (Dunno what the differences between versions are.)
Free Pascal / Lazarus is a FOSS cross-platform language and GUI IDE. I'd recommend trying that in stead.
