[MacPorts] #57694: clang-6.0: build fails in compiler_rt on < SnowLeopard due to fatal error: 'dispatch/dispatch.h' file not found

[MacPorts]

#57694: clang-6.0: build fails in compiler_rt on < SnowLeopard due to fatal error:
'dispatch/dispatch.h' file not found
------------------------+---------------------------
  Reporter:  kencu      |      Owner:  jeremyhu
      Type:  defect     |     Status:  assigned
  Priority:  Normal     |  Milestone:
 Component:  ports      |    Version:
Resolution:             |   Keywords:  tiger leopard
      Port:  clang-6.0  |
------------------------+---------------------------
Changes (by kencu):

 * Attachment "ken_os_version_check_clang-6.c" added.

 my patched version of os_version_check.c with a small test prog added on
 the bottom

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Ticket URL: <https://trac.macports.org/ticket/57694>
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