build: make graphviz optional for building documentation

Change-Id: I5434bbef22c840341623bff16f9e8de129249be2

docs/doxygen.conf.in

@@ -230,12 +230,6 @@ TAB_SIZE               = 4
 
 ALIASES                =
 
-# This tag can be used to specify a number of word-keyword mappings (TCL only).
-# A mapping has the form "name=value". For example adding "class=itcl::class"
-# will allow you to use the command class in the itcl::class meaning.
-
-TCL_SUBST              =
-
 # Set the OPTIMIZE_OUTPUT_FOR_C tag to YES if your project consists of C sources
 # only. Doxygen will then generate output that is more tailored for C. For
 # instance, some of the names that are used will be different. The list of all
@@ -2162,7 +2156,7 @@ HIDE_UNDOC_RELATIONS   = YES
 # set to NO
 # The default value is: YES.
 
-HAVE_DOT               = YES
+HAVE_DOT               = @HAVE_DOT@
 
 # The DOT_NUM_THREADS specifies the number of dot invocations doxygen is allowed
 # to run in parallel. When set to 0 doxygen will base this on the number of

wscript

@@ -92,6 +92,7 @@ def configure(conf):
     conf.env.WITH_OTHER_TESTS = conf.options.with_other_tests
 
     conf.find_program('bash', var='BASH')
+    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))
@@ -267,6 +268,7 @@ def doxygen(bld):
         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',