build: align minimum build dependencies with ndn-cxx

 * gcc 7.4
 * clang 6.0
 * Xcode 10.0 (11.3 or later recommended)

Also update waf to version 2.0.23

Change-Id: I893ac70b8a64ec4155bd416b2ac01e9b3ff0aa29
diff --git a/wscript b/wscript
index 09a0147..6e6974f 100644
--- a/wscript
+++ b/wscript
@@ -62,9 +62,14 @@
                 source='src/main.cpp',
                 use='repo-objects')
 
-    bld.recurse('tools')
-    bld.recurse('tests')
-    bld.recurse('examples')
+    if bld.env.WITH_TOOLS:
+        bld.recurse('tools')
+
+    if bld.env.WITH_TESTS:
+        bld.recurse('tests')
+
+    if bld.env.WITH_EXAMPLES:
+        bld.recurse('examples')
 
     bld.install_files('${SYSCONFDIR}/ndn', 'repo-ng.conf.sample')