use feature test macros instead of version checks

[benedekkupper]

I have started working on #572 myself, the branch is currently work in progress, so don't consider this a real PR at the moment, more of an indication that there's something being done in this direction.

My workflow is this:

1. git grep ETL_USING_CPP
2. Try to deduct what feature is needed from that version of C++.
3. Look up the macro name for the feature on https://en.cppreference.com/w/cpp/feature_test
4. If found, replace ETL_USING_CPPxx with the feature test macro, otherwise add a comment to determine_compiler_language_support.h

[jwellbelove]

I'm not sure how these can be used before C++20.
e.g.
How can you use C++20's __cpp_rvalue_references in place of ETL_USING_CPP11?
In a compiler only supporting, C++11 __cpp_rvalue_references will be undefined.

[jwellbelove]

As your link mentions, these feature test macros are only defined for compilers that support C++20 or above.

Feature testing (since C++20) <<<<<<
Feature testing
The standard defines a set of preprocessor macros corresponding to C++ language and library features introduced in C++11 or later. They are intended as a simple and portable way to detect the presence of said features.

[benedekkupper]

I have backported these feature test macros into determine_compiler_language_support.h, see ETL_NO_STD_FEATURE_TESTS section. There is a <version> header defined in C++20 that only contains these macros, hopefully the compilers that add support for this in their C++20 version, also add this header to their earlier version lib as well. Even if not, I have added the macros that are used by ETL (also some notes on what macros are missing, etc).

Using these macros gives a better idea on what compiler features are required, and possibly extend the compiler support in case a compiler version only implements a subset of a C++ version (e.g. the current arm gcc C++20 version is missing a bunch of C++20 functionality, so just checking for the C++ version enables code sections that don't actually have compiler support yet.