They're no longer used.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Instead of globbing things under an ambiguous Common.h header, move
compiler-specifics over to Compiler.h. This gives us a dedicated home
for anything related to compilers that we want to make functional across
all compilers that we support.

This moves us a little closer to eliminating Common.h entirely.

Given this is actually a part of the Host interface, this should be
placed with it.

While we're at it, turn it into an enum class so that we don't dump its
contained values into the surrounding scope. We can also make
Host_Message take the enum type itself directly instead of taking a
general int value.

After this, it'll be trivial to divide out the rest of Common.h and
remove the header from the repository entirely

This was added in 4bdb4aa0d1c8520488583ba79c9f5ab6e5656eca back in
2009-02-27. The only usage spot of this macro involves the same checks
that were used to define that preprocessor macro, so we can simply
remove the macro

Ideally Common.h wouldn't be a header in the Common library, and instead be renamed to something else, like PlatformCompatibility.h or something, but even then, there's still some things in the header that don't really fall under that label

This moves the version strings out to their own version header that doesn't dump a bunch of other unrelated things into scope, like what Common.h was doing.

This also places them into the Common namespace, as opposed to letting them sit in the global namespace.

I think we don't have to double check it when building the WX target.