[MacPorts] #54129: llvm-4.0 build fails due to libtool error on SnowLeopard when built with clang-3.9, but succeeds when built with clang-3.7

[MacPorts]

#54129: llvm-4.0 build fails due to libtool error on SnowLeopard when built with
clang-3.9, but succeeds when built with clang-3.7
-----------------------+----------------------
  Reporter:  kencu     |      Owner:  jeremyhu
      Type:  defect    |     Status:  accepted
  Priority:  Normal    |  Milestone:
 Component:  ports     |    Version:
Resolution:            |   Keywords:
      Port:  llvm-4.0  |
-----------------------+----------------------

Comment (by Ionic):

 This doesn't seem to work as intended.

 For some reason, the code that adds the `cctools` dependency and checks
 whether `cctools` have been added as a dependency to add the
 `cmake`/`configure` option is executed before compiler blacklisting takes
 place - which means that `gcc-4.2` is still selected as the compiler and
 everything fails badly.

 If I wrap both checks into a `pre-configure` block, delaying execution, it
 starts working as intended.

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Ticket URL: <https://trac.macports.org/ticket/54129#comment:23>
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