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:

  GTK3=yes  link with gtk+3 (default)
  GTK3=no   link with gtk+2
  GTK3=-    don't link against gtk at all

Pass the script the parameter you prefer, like
  GTK3=no ./libfm.SlackBuild