[MacPorts] #50800: cairo, cairo-devel: fix for build failures against Xcode 7.3 linker when /usr/local/lib is absent

[MacPorts]

#50800: cairo, cairo-devel: fix for build failures against Xcode 7.3 linker when
/usr/local/lib is absent
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  Reporter:  howarth.at.macports@…  |      Owner:  ryandesign
      Type:  defect                 |     Status:  new
  Priority:  Normal                 |  Milestone:
 Component:  ports                  |    Version:  2.3.4
Resolution:                         |   Keywords:  haspatch
      Port:  cairo, cairo-devel     |
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Comment (by kencu):

 one useful thing to consider is that adding `-Wl,-w` to the build ldflags
 appears to suppress warnings coming out the linker.

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Ticket URL: <https://trac.macports.org/ticket/50800#comment:5>
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