Enable CMake Unity build for Android

[louwers]

When this property is set to true, the target source files will be combined into batches for faster compilation. This is done by creating a (set of) unity sources which #include the original sources, then compiling these unity sources instead of the originals. This is known as a Unity or Jumbo build.

https://cmake.org/cmake/help/latest/prop_tgt/UNITY_BUILD.html

Experiment to allow for making unity builds with CMake. This has the potential to significantly speed up build times.

Edit: looks like for Android libmaplibre.so compilation time goes from 2m 6s to 53s.

[ntadej]

Can you maybe provide a bit of a description what is that and why is it useful? 😊

[louwers]

@ntadej Yeah I just opened it to see if it builds. 🙈

[github-actions]

Bloaty Results (iOS) 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%    +264  [ = ]       0    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results-ios/pr-2932-compared-to-main.txt

[github-actions]

Bloaty Results 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  -0.0% -20.6Ki  -0.0% -7.17Ki    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-2932-compared-to-main.txt

Compared to d387090 (legacy)

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +28% +32.6Mi  +426% +25.4Mi    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-2932-compared-to-legacy.txt