build: Fix Qt processing of configure script for depends with DEBUG=1

Now, if depends is built with DEBUG=1, the configure script correctly
finds Qt for macOS and Windows.

depends/config.site.in

@@ -54,6 +54,10 @@ if test -z "$with_gui" && test -n "@no_qt@"; then
   with_gui=no
 fi
 
+if test -n "@debug@" && test -z "@no_qt@" && test "x$with_gui" != xno; then
+  with_gui=qt5_debug
+fi
+
 if test -z "$enable_zmq" && test -n "@no_zmq@"; then
   enable_zmq=no
 fi