[MacPorts] #44381: Add port:flex to graphviz build dependencies

[MacPorts]

#44381: Add port:flex to graphviz build dependencies
-------------------------+--------------------------------------
 Reporter:  michaelld@…  |      Owner:  ryandesign
     Type:  defect       |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:  2.3.1
 Keywords:               |       Port:  graphviz, graphviz-devel
-------------------------+--------------------------------------
 graphviz checks for flex, and uses the system version (/usr/bin/flex) if
 found. If the flex port is installed, it will use that -- but, flex is not
 listed as a build dependency. At least on some systems (per a customer
 support issue I was handling for GNU Radio), the system flex fails to
 property generate output, causing graphviz to fail to configure. Moving to
 using MacPorts' flex solves this issue. Hence, adding in port:flex as a
 build dependency seems like the way to go.

-- 
Ticket URL: <https://trac.macports.org/ticket/44381>
MacPorts <http://www.macports.org/>
Ports system for OS X