[MacPorts] #41154: openscad: fix build with libc++

[MacPorts]

#41154: openscad: fix build with libc++
------------------------+-------------------------------
  Reporter:  ecronin@…  |      Owner:  macsforever2000@…
      Type:  defect     |     Status:  new
  Priority:  Normal     |  Milestone:
 Component:  ports      |    Version:  2.2.1
Resolution:             |   Keywords:  mavericks libc++
      Port:  openscad   |
------------------------+-------------------------------

Comment (by jeremyhu@…):

 The best approach is to just make the code portable, but if that's not
 possible, this will suffice:

 {{{
 #include <ciso646>
 #ifdef _LIBCPP_VERSION
 // libc++
 #else
 // libstdc++
 #endif
 }}}

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Ticket URL: <https://trac.macports.org/ticket/41154#comment:1>
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