Check for Qt depends in LOCALBASE, not PREFIX

Qt has been looking for its dependencies in PREFIX, rather than in
LOCALBASE. Dependencies are expected to exist in LOCALBASE, but
PREFIX may be set arbitrarily by the end-user. When PREFIX != LOCALBASE,
Qt-based ports failed to build.

PR: 222994
Reported by: kargl
Approved by: tcberner (kde)
Exp-run by: antoine
MFH: 2017Q4
Differential Revision: https://reviews.freebsd.org/D12666