I've posted a book on the site that will show you how to quickly set up IUP on your Linux box.
IUP is an amazingly simple to use, modern GUI toolkit. It can be completely programmed in C or Lua. Many users use it for C++ as well. It runs over GTK, Win32, and Motif.
The full project page is here:
However, here is my Quick and Dirty Guide for IUP on Linux systems:
Download a 3.0rc3 executable (or more current) for Linux that fits your arch and the GCC version of your libs (or whatever you might link into your app). The IUP project pages will link you over to SourceForge.
Please note that the Linux binaries are not in the folder that says "Newest Files" or even "Linux Binaries"....those are the libs by themselves. How cryptic is that? Well, it reflects the notion that lots of IUP users use the C API. In fact, the tarball I'll direct you to below has those same .so libs, as well as a useful app in C that lets you load Lua scripts quickly.
Get your tarball from the "Tools Executables" dir on SourceForge.