[MacPorts] #44067: use "configure.includes {path1 path2}" instead of "configure.cppflags -I{path1} -I{path2}"

[MacPorts]

#44067: use "configure.includes {path1 path2}" instead of "configure.cppflags
-I{path1} -I{path2}"
-------------------------+--------------------------------
 Reporter:  mojca@…      |      Owner:  macports-tickets@…
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  base         |    Version:  2.3.0
 Keywords:               |       Port:
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 Given that some build systems like CMake don't directly support `CPPFLAGS`
 (see #42872) (but also for other reasons) it would be nice to specify the
 include paths with an array of paths as opposed to "hardcoded"
 `configure.cppflags -I{path1} -I{path2} -I{path3}`.

 The advantage of this approach would be that one could easily switch to
 `-isystem {path1} -isystem {path2}` or set
 `CMAKE_INCLUDE_PATH={path1}:{path2}` or whatever, depending on OS version,
 build system etc.

 A similar flag would be nice for library paths ('-L/opt/local/lib') and
 framework paths (`-F/opt/local/Library/Frameworks`).

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Ticket URL: <https://trac.macports.org/ticket/44067>
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