[MacPorts] #55949: libpng.pc should have zlib in Requires.private

[MacPorts]

#55949: libpng.pc should have zlib in Requires.private
--------------------+------------------------
 Reporter:  jmroot  |      Owner:  ryandesign
     Type:  defect  |     Status:  new
 Priority:  Normal  |  Milestone:
Component:  ports   |    Version:
 Keywords:          |       Port:  libpng
--------------------+------------------------
 Having it in Requires causes overlinking in dependents:
 {{{
 % pkg-config --libs libpng
 -L/opt/local/lib -lpng16 -lz
 }}}

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Ticket URL: <https://trac.macports.org/ticket/55949>
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