#2148 CMake 3.15

[Mykola Orliuk]

Originally at https://github.com/FlightGear/simgear/pull/2

Fixes next error:

CMake Error at CMakeModules/BoostTestTargets.cmake:55 (message):                                                                                                                          
  NOTE: boost::test-based targets and tests cannot be added: boost >= 1.34.0                                                                                                              
  required but not found.  (found: '1.71.0' want >=103400) You may disable                                                                                                                
  ENABLE_TESTS to continue without the tests.                                                                                                                                             
Call Stack (most recent call first):                                                                                                                                                      
  CMakeLists.txt:220 (include) 

[James Turner]

We're discussoing work arounds to this on the mailing list at the moment.

[Mykola Orliuk]

Great. Found it https://sourceforge.net/p/flightgear/mailman/message/36778139/ .
Note that this change should be compatible with old CMake.

[Mykola Orliuk]

I see discussion there right now is going around switching that policy to OLD behavior. Please not that it is already supposed to be turned to that state by default in 3.15.4 as per https://cmake.org/cmake/help/v3.15/policy/CMP0093.html . But for some reason in 3.15.3 it is turned to NEW and attempt to switch to OLD didn't work for me (neither by in-cmake statement nor via specifying -DCMAKE_POLICY_DEFAULT_CMP0093=OLD).

As far as I understand those policies supposed to warn about using default OLD when you don't turn them to NEW (though this partgicular should silently use OLD). I guess minimal version of cmake specified in CMakeLists.txt also have some effect on when that warning emitted. But in my case (3.15.3) I didn't got any warnings rather than just NEW behavior.

[James Turner]

I've pushed your change becuase I have the same problem in my CMake install (NEw is the default, and OLD doesn't work).

I have also asked about this on the CMake list, and so far no one answered.