Use base zlib.  A future version of qcad might need this patch moved
from 3rdparty/legacy/opennurbs to 3rdparty/opennurbs.

Index: src/3rdparty/legacy/opennurbs/opennurbs.pro
--- src/3rdparty/legacy/opennurbs/opennurbs.pro.orig
+++ src/3rdparty/legacy/opennurbs/opennurbs.pro
@@ -1,5 +1,4 @@
 include (../../../../shared.pri)
 TEMPLATE = subdirs
 SUBDIRS = \
-    zlib \
     opennurbs
