[MacPorts] #40656: Use -isystem instead of -I in default configure.cppflags

[MacPorts]

#40656: Use -isystem instead of -I in default configure.cppflags
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  Reporter:  ryandesign   |      Owner:  macports-tickets@…
      Type:  enhancement  |     Status:  reopened
  Priority:  Normal       |  Milestone:  MacPorts Future
 Component:  base         |    Version:  2.2.0
Resolution:               |   Keywords:  haspatch
      Port:               |
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Comment (by mohd-akram):

 CPATH seems to make sense to me too and should be sufficient. I personally
 set CPATH and LIBRARY_PATH in my shell profile to the MacPorts paths for
 building external projects and haven't experienced any issues with that.

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Ticket URL: <https://trac.macports.org/ticket/40656#comment:23>
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