Fixes to spec file
    
    - Use macros for paths instead of absolute paths.
    - Remove dangling %{gtk_arg} macro in configure.
    - Fix scope of enable_autotools macro to avoid warning during build.
      warning: Macro %enable_autotools defined but not used within scope
    
    Signed-off-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>