[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:  closed
  Priority:  Normal        |  Milestone:  MacPorts Future
 Component:  base          |    Version:  2.2.0
Resolution:  fixed         |   Keywords:  haspatch
      Port:                |
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Comment (by jmr@…):

 This changes the include order in subtle ways: `-I${prefix}/include
 -I${prefix}/include/some_subdir` is different to
 `-isystem${prefix}/include -I${prefix}/include/some_subdir`. Definite
 potential for confusing breakage.

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Ticket URL: <https://trac.macports.org/ticket/40656#comment:12>
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