[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:  defect        |     Status:  new
 Priority:  Normal        |  Milestone:  MacPorts Future
Component:  base          |    Version:  2.2.0
 Keywords:  haspatch      |       Port:
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 We should [https://lists.macosforge.org/pipermail/macports-
 dev/2013-October/024536.html consider changing the default
 configure.cppflags] from "`-I`" to "`-isystem`".
 [http://gcc.gnu.org/onlinedocs/gcc/Directory-Options.html#Directory-
 Options -isystem is similar to -I] except that the given directory is
 checked after all "`-I`" directories are checked. In this way, using
 "`-isystem`" instead of "`-I`" would fix a whole class of problems where
 some ports sometimes fail to upgrade when a previous version is installed.
 Several ports (including freetype, ghostscript, pianobar) already make
 this change, so making this change in base would allow us to simplify
 those ports. The attached patch changes "`-I`" to "`-isystem`" everywhere
 I could find it.

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