cmake: refactor game build configuration to reuse the same code in parent build or subproject build and mutualize code between vm type build

[illwieckz]

Refactor game build configuration to reuse the same code in parent build or subproject build and mutualize code between vm type build.

Extracted from:

• Refactor game build configuration and build dll as subproject #1484

This one brings nothing new, it just refactors the code in a better way so it makes future changes easier.

Amont various things,

• it makes building vm subproject independent from NaCl
• it deduplicates and mutualises the code to avoid maintaining multiple copy-paste

[illwieckz]

There is no need for game-side patch anymore (which is now closed):

• cmake: refactor game build configuration to reuse the same code in parent build or subproject build and mutualize code between vm type build Unvanquished/Unvanquished#3318

This PR is standalone.

[illwieckz]

Now rebased.

[slipher]

LGTM