enable unity builds

[vrcomputing]

What

Enable CMAKE_UNITY_BUILD accelerating total build time from 50s to 36s. 😮

This enables unity builds globally ( ⚠️ not recommended )

set(CMAKE_UNITY_BUILD ON)

This enables unity builds on a specific target

set_target_properties(${target} PROPERTIES UNITY_BUILD ON)

This excludes generated moc_* files from it

if(BUILD_UNITY)
    file(GLOB_RECURSE MOC_FILES
        ${CMAKE_CURRENT_BINARY_DIR}/moc_*.cpp
    )

    set_source_files_properties(${MOC_FILES} PROPERTIES SKIP_UNITY_BUILD_INCLUSION ON)
endif()