cmake: build hardened dll game with sub cmake to use different compilation flags

[illwieckz]

Build dll game with sub cmake to use different compilation flags.

Obsoletes:

• cmake: make possible to enable LTO when compiling dll games #1481

See on game side:

• cmake: build hardened dll game with sub cmake to use different compilation flags Unvanquished/Unvanquished#3270

[slipher]

Please no 😭. Having stuff built in a sub-invocation makes everything suck. For example:

• When building with Ninja you don't get any progress messages.
• Can't build targets individually. I often use this ability when working on something in the sgame because of the cgame taking a lot longer to build (due to rmlui).

[illwieckz]

Now it only builds the dll game in a subproject when HARDENING is ON, it builds it as a usual otherwise.

I refactored a lot the code, also fixed some mistakes with nexe (the nacl target strings was parsed also with saigo, but there is none). Actually the build* functions I've done may be refactored too but I postpone that.

[illwieckz]

Actually the build* functions I've done may be refactored too but I postpone that.

Rethinking about it, I discovered it was easier than what I thought, so I did it.

[illwieckz]

I'm happy with that deep refactor. It will also make implementing Wasm easier.

I split the changes in two commits:

• The deep refactor making it easy to build something as subproject without copy-pasting a ton of boiler-plate.
• A small commit building the dll as subproject when USE_HARDENED is enabled, buit building the dll as usual otherwise.

The same code now being reused for the various builds, it is even more robust than ever.