add NDNCERT_prefix and small fixes

Change-Id: I7ad5e3258599e62cce437555e64a3b461b08f8e6
diff --git a/wscript b/wscript
index 4204721..c5df199 100644
--- a/wscript
+++ b/wscript
@@ -52,7 +52,7 @@
     # system has a different version of the ndncert library installed.
     conf.env.prepend_value('STLIBPATH', ['.'])
 
-    conf.define_cond('HAVE_TESTS', conf.env.WITH_TESTS)
+    conf.define_cond('NDNCERT_HAVE_TESTS', conf.env.WITH_TESTS)
     conf.define('SYSCONFDIR', conf.env.SYSCONFDIR)
     # The config header will contain all defines that were added using conf.define()
     # or conf.define_cond().  Everything that was added directly to conf.env.DEFINES