build: make graphviz optional for building documentation

Change-Id: I779e69037ecef1d49e035a572a843ef3a861e614
diff --git a/wscript b/wscript
index d6e64d7..89287e8 100644
--- a/wscript
+++ b/wscript
@@ -39,6 +39,8 @@
     conf.env.WITH_EXAMPLES = conf.options.with_examples
     conf.env.WITH_TESTS = conf.options.with_tests
 
+    conf.find_program('dot', var='DOT', mandatory=False)
+
     conf.check_cfg(package='libndn-cxx', args=['--cflags', '--libs'], uselib_store='NDN_CXX',
                    pkg_config_path=os.environ.get('PKG_CONFIG_PATH', '%s/pkgconfig' % conf.env.LIBDIR))
 
@@ -119,6 +121,7 @@
         target=['docs/doxygen.conf',
                 'docs/named_data_theme/named_data_footer-with-analytics.html'],
         VERSION=VERSION,
+        HAVE_DOT='YES' if bld.env.DOT else 'NO',
         HTML_FOOTER='../build/docs/named_data_theme/named_data_footer-with-analytics.html' \
                         if os.getenv('GOOGLE_ANALYTICS', None) \
                         else '../docs/named_data_theme/named_data_footer.html',