Build system not mentioned
Check out cmake (http://www.cmake.org/) sometime. I've recently started using it for my projects, and I've found it to be very flexible.
Re: Licensing issue leaves me confused.
The difference is, GTK is under the LGPL, not the GPL. The LGPL (Lesser GPL) states that it's OK to link to the library, under whatever license you choose. This is how someone could make a non-free program that used GTK as it's widget set.