IDL43 — Feature macros to guard building blocks

Now that IDL is modular with building blocks, IDL authors need a way of guarding IDL files so they are compatible with tools that implement different subsets of the building blocks. See discussion in IDL43-21 for a motivating example.

See C++ feature test macros for a possible implementation pattern.
https://en.cppreference.com/w/cpp/feature_test

Another option is to have an "IDL_VERSION" macro that can be used as a pre-processor condition.