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

 When building MacPorts itself, we should stick to the usual `-I`. I don't
 see any advantage of this change there.

 Be aware that using `-isystem` hides all warnings from the header files as
 they are treated as if they were system headers. I worry this could mask
 problems during building and make it harder to identify the cause.

 There is also the environment variable `CPATH` for additional include
 paths as if they were given using `-I` on the command line
 ([gcc.gnu.org/onlinedocs/gcc/Environment-Variables.html gcc manual]).
 However, older versions of clang do not honor this setting.

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Ticket URL: <https://trac.macports.org/ticket/40656#comment:4>
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