add ACF_USE_DRISHTI_CACHE optino : to {en,dis}able drishti-upload submodule config and associated github hunter-cache; else default to minimal local config for recent hunter packages
remove unused DRISHTI_IS_MOBILE (copy and paste)
add ACF_BUILD_EXAMPLES to control src/app builds
set ACF_BUILD_TESTS=OFF by default and enable explicltly in ci build tests
only use half package when cereal serialization is enabled