SPQR: Make GPUQREngine and SuiteSparse_GPURuntime subprojects #461

Job	Run time
macos	57m 24s
msvc (with OpenMP)	7m 50s
mingw (MINGW64)	10m 9s
msvc (without OpenMP)	6m 6s
mingw (MINGW32)	10m 14s
mingw (CLANG64)	1h 20m 26s
ubuntu (gcc with CUDA with OpenMP)	11m 26s
mingw (CLANG32)	1h 39m 40s
ubuntu (gcc without CUDA with OpenMP)	3m 30s
ubuntu (clang with CUDA with OpenMP)	12m 43s
ubuntu (clang without CUDA with OpenMP)	1h 46m 38s
ubuntu (gcc without CUDA without OpenMP)	3m 12s
	6h 49m 18s

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