During the transition from Mk/bsd.qt.mk to Mk/Uses/qt.mk in r473503, a previous change to the former was omitted; in particular, r463394:
"Disable Qt4 sql-ibase plugin on not-(i386|amd64) ibase is Firebird, which only builds on i386 and amd64. This patch squashes the ibase plugin for Qt4."
Re-apply lost change (databases/qt5-sqldrivers-ibase is x86-only).
Submitted by: linimon