xfce-do-release: FreeBSD fixes
Fixed following FreeBSD related issues:
- xfce-do-release: FreeBSD 'sed' require that zero-length extension is specified when -i switch enabled
- xfce-do-release: added detection for 'gmake'
- xdt-autogen: added correct 'make' to last displayed message
This MR is not tested on Linux.
This MR fixes issues listed in #59 (closed).