cmake: Use target_compile_features()

Problem at hand: dependents (like FairRoot) need to know the
minimum C++ standard level that FairMQ (and its headers)
requires.

The first idea is to let the targets export their
CXX_STANDARD value. First, this doesn't seem to work as
expected.

Second, target_compile_features() seems to be the better
way to go. It has a much better granularity, automatically
has the export feature, and thus should make dependents
behave correctly.

Also drop all of this enforeced CMAKE_CXX_STANDARD*
setting. If it's given, check it. But that's it.

See: https://gitlab.kitware.com/cmake/cmake/-/issues/18446
See: https://cmake.org/cmake/help/latest/manual/cmake-compile-features.7.html#requiring-language-standards

cmake/FairMQLib.cmake

@@ -94,10 +94,7 @@ macro(set_fairmq_defaults)
   endif()
 
   # Handle C++ standard level
-  set(CMAKE_CXX_STANDARD_REQUIRED ON)
-  if(NOT CMAKE_CXX_STANDARD)
-    set(CMAKE_CXX_STANDARD ${PROJECT_MIN_CXX_STANDARD})
-  elseif(${CMAKE_CXX_STANDARD} LESS ${PROJECT_MIN_CXX_STANDARD})
+  if(CMAKE_CXX_STANDARD AND CMAKE_CXX_STANDARD VERSION_LESS PROJECT_MIN_CXX_STANDARD)
     message(FATAL_ERROR "A minimum CMAKE_CXX_STANDARD of ${PROJECT_MIN_CXX_STANDARD} is required.")
   endif()
   set(CMAKE_CXX_EXTENSIONS OFF)