Refactor SConstruct, and add scu_build option. #51

Job	Run time
build (linux, single, x86_64, template_debug, ubuntu-20.04)	30s
build (windows, single, x86_32, template_debug, windows-latest)	10m 3s
build (windows, single, x86_64, template_debug, windows-latest)	9m 38s
build (macos, single, universal, template_debug, macos-latest)	43s
build (android, single, arm64, template_debug, ubuntu-20.04)	1m 16s
build (android, single, arm32, template_debug, ubuntu-20.04)	1m 14s
build (android, single, x86_64, template_debug, ubuntu-20.04)	2m 17s
build (android, single, x86_32, template_debug, ubuntu-20.04)	1m 23s
build (ios, single, arm64, template_debug, macos-latest)	51s
build (web, single, wasm32, template_debug, ubuntu-20.04)	32s
build (linux, single, x86_64, template_release, ubuntu-20.04)	28s
build (linux, double, x86_64, template_debug, ubuntu-20.04)	29s
build (windows, double, x86_32, template_debug, windows-latest)	9m 43s
build (windows, double, x86_64, template_debug, windows-latest)	4m 3s
build (macos, double, universal, template_debug, macos-latest)	44s
build (android, double, arm64, template_debug, ubuntu-20.04)	1m 11s
build (android, double, arm32, template_debug, ubuntu-20.04)	2m 12s
build (android, double, x86_64, template_debug, ubuntu-20.04)	1m 18s
build (android, double, x86_32, template_debug, ubuntu-20.04)	3m 4s
build (ios, double, arm64, template_debug, macos-latest)	1m 11s
build (web, double, wasm32, template_debug, ubuntu-20.04)	29s
build (linux, double, x86_64, template_release, ubuntu-20.04)	28s
merge	6s
	53m 53s

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