build system: remove the unused DEP variable
build system: remove the unused DEP variable

It's some kind of obfuscating fossil.

ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror