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

fairmq/sdk/CMakeLists.txt

@@ -45,6 +45,7 @@ add_library(${target}
   ${SDK_PUBLIC_HEADER_FILES} # for IDE integration
   ${SDK_PRIVATE_HEADER_FILES} # for IDE integration
 )
+target_compile_features(${target} PUBLIC cxx_std_17)
 set_target_properties(${target} PROPERTIES LABELS coverage)
 target_include_directories(${target}
   PUBLIC