[MacPorts] #53615: cmake : ignore policy CMP0025

[MacPorts]

#53615: cmake : ignore policy CMP0025
----------------------+-------------------
 Reporter:  RJVB      |      Owner:
     Type:  defect    |     Status:  new
 Priority:  Normal    |  Milestone:
Component:  ports     |    Version:
 Keywords:  haspatch  |       Port:  cmake
----------------------+-------------------
 CMake has a policy (number 25) under which it considers that only Apple's
 clang can provide information on the features it supports. Depending on
 how the CMake file is configured this can lead to the following vague
 error (printed *after* the whole configure process has been completed),
 when using a clang version from MacPorts:

 {{{
 -- Configuring done
 CMake Error in CMakeLists.txt:
   No known features for CXX compiler

   "Clang"

   version 3.9.0.
 }}}

 There are ways around this by patching the CMake file, and I have seen
 some limited success in adding a default cmake argument
 (`-DCMAKE_POLICY_DEFAULT_CMP0025=NEW`) but ultimately this depends on a
 few rather silly checks that IMHO could just as well be disabled at the
 source. The attached patch does that.

 NB: the CMake files affected by this aren't erroneous, in my opinion CMake
 is wrong in handling AppleClang differently

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Ticket URL: <https://trac.macports.org/ticket/53615>
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