Convert bsd.gnustep.mk to USES=gnustep

Simplify gnustep ports
Hook into the regular ports framework:

• LIB_DEPENDS for library dependencies
• Use regular USE_LDCONFIG

Reuse USES=objc (automatic)
USE_GNUSTEP is now a macro to set the dependencies and build feature needed.
Accepted arguments: back base build gui

Merge deskutils/preferencepanes into deskutils/systempreferences