LibFM provides file management functions built on top of Glib/GIO, 
giving a convenient higher-level API.

You can choose which version of gtk the library will build against:

  GTK=2  link with gtk+2 (default)
  GTK=3  link with gtk+3
  GTK=0  don't link against gtk at all

Pass the script the parameter you prefer, like
  GTK=3 ./libfm.SlackBuild