Fix up logic to set minimum C++ standard.

This version does not enforce a downgrade if the default standard in use on a system is higher.
This relates to PR #134 and discussion in TDE/tqt#244.

Signed-off-by: Michele Calgaro <michele.calgaro@yahoo.it>
(cherry picked from commit 34d852f2df)

modules/TDEMacros.cmake

@@ -2926,9 +2926,7 @@ macro( tde_setup_compiler_flags )
 
   # Set minimum required ISO C++ standard and forces
   # failure in case the compiler doesn't support it
-  set(CMAKE_CXX_STANDARD 17)
-  set(CMAKE_CXX_STANDARD_REQUIRED ON)
-  set(CMAKE_CXX_EXTENSIONS OFF)
+  set( TDE_CXX_FEATURES cxx_std_17 )
 
 endmacro( )