doxygen: add graphviz variant; enable flex and bison on OSX and they build fine

var/spack/repos/builtin/packages/doxygen/package.py

@@ -22,10 +22,6 @@
 # License along with this program; if not, write to the Free Software
 # Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
-
-# Author: Justin Too <justin@doubleotoo.com>
-# Date: September 11, 2015
-
 from spack import *
 import sys
 
@@ -43,10 +39,15 @@ class Doxygen(Package):
     version('1.8.11', 'f4697a444feaed739cfa2f0644abc19b')
     version('1.8.10', '79767ccd986f12a0f949015efb5f058f')
 
+    # graphviz appears to be a run-time optional dependency
+    variant('graphviz', default=True, description='Build with dot command support from Graphviz.')
+
     depends_on("cmake@2.8.12:")
-    # flex does not build on OSX, but it's provided there anyway
-    depends_on("flex", sys.platform != 'darwin')
-    depends_on("bison", sys.platform != 'darwin')
+    depends_on("flex")
+    depends_on("bison")
+
+    #optional dependencies
+    depends_on("graphviz", when="+graphviz")
 
     def install(self, spec, prefix):
         cmake('.', *std_cmake_args)