azureus-bin accept "gtk" keyword, however, it actually uses gtk2. so that the keyword should be "gtk2". Reproducible: Always Steps to Reproduce: 1. 2. 3.
gtk2 use flag is only used when the program has support for both GTK1 and GTK2 and have to choose which to use. If azureus-bin only supports gtk2 then gtk is the right flag.
Soren is correct