Lazarus is a Rapid Application Development Tool for FreePascal.
It comes with the LCL - Lazarus component library, which contains platform
independent visual components like buttons, windows, checkbox, treeview and
many, many more. The LCL is platform independent, so you can write an
application once and then compile for various platforms without changing code.

Note: To build lazarus with qt4 interface (LCL) you need to install qt4pas
and define the variable LCL:   LCL=qt ./lazarus.SlackBuild

Note: This won't build on non-i386 platforms.

This requires fpc [Free pascal].