Ported library related commits (and 1 minor for readme) from 0.8.1 build + my commits to add more strictness to zandronum

[deguix]

Tested against DnD's c252095f7c03ee6efc64c1946b9ca4349fe7d736 commit before it got updated to use 0.8.1 features. Everything succeeds. Might need to remove stuff related to things not supported (like the language removal support) and need to check for gzdoom support, but it includes a missed fix for it.

[TDRRmk2]

I'm on board with this PR, mostly.
Only thing I'd change is the way the engine is specified. A macro is a bit ugly. And ideally by default, all functions should be included, for SLADE compatibility (and because this is the case for the other BCS features changing behavior, they're all opt-in except for short circuiting).

There should be a way (maybe a pseudo preprocessor directive) to specify an engine in-source, that doesn't need to be before the zcommon include, and only then exclude the other functions. A way to set this on the command line as well.

Not just using a macro would be nice in case of engine-specific quirks as well. Eternity can have delays in functions so that should be allowed when it's the target engine, as an example.

I'll take a look into doing all this and if I can do it, I'll merge this PR. Thank you.

[deguix]

Please recheck, I made a mistake when getting everything into 1 file, and you commented right when I was trying to fix it. I found out later that getting everything into 1 file doesn't work because it seems bcc has a feature to import only certain functions (and things like macros don't get imported).