Are there any high-level, cross-platform, and abstracted (simple to use) widget toolkits for C that are either very lightweight and fast or can be compiled for other toolkits, such as Qt and GTK+?
EDIT:
Most C++ libraries lack C bindings, so they will be of little (or no) help.
enlightenment enables real cross platform from Windows CE to WIndows 7, from Linux to embedded Linux, Mac OS to Mac OS X and it is leightwieght
http://www.enlightenment.org