lang/smalltalk: apply some minor corrections to the port's Makefile (+)

• Restore linking against `math/gmp' which had been disabled in r73446 (now commit ae50e2b416e79) without an explanation
• Unfold PORTDATA=* back to static plist entries by partially reverting r253699 (now commit ff234fed369c3) to help track, admittedly, stalled project's development and assess the changes better
• Trim USE_XORG list, switch to USE_GNOME=cairo, sort LIB_DEPENDS, etc.