recent changes caused a compile issue with QT

On a machine with both homebrew and macports, the presence of
macports was causing the homebrew Qt settings to be overwritten
right after they were set. Check to see if QTDIR is already
defined before proceeding with QT macports.

sys/unix/hints/macOS.370

@@ -63,6 +63,7 @@ ifdef WANT_WIN_QT6
 QTDIR :=$(shell brew --prefix)/opt/qt@6
 endif   # WANT_WIN_QT6
 endif   # HAVE_HOMEBREW
+ifndef QTDIR
 ifeq "$(HAVE_MACPORTS)" "1"
 ifdef WANT_WIN_QT5
 # Qt5 installed via macports
@@ -72,6 +73,7 @@ ifdef WANT_WIN_QT6
 QTDIR=/opt/local/libexec/qt6
 endif   # WANT_WIN_QT6
 endif   # HAVE_MACPORTS
+endif   # QTDIR
 endif	# WANT_WIN_QT
 
 ifndef LIBXPM