cmake: build 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 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).