LAGraph: Link to OpenMP::OpenMP_C target #477

Job	Run time
msvc (with OpenMP)	5m 46s
mingw (MINGW64)	7m 53s
msvc (without OpenMP)	6m 56s
ubuntu (gcc with CUDA with OpenMP)	7m 8s
mingw (MINGW32)	8m 18s
macos	1h 36m 23s
ubuntu (gcc without CUDA with OpenMP)	3m 44s
mingw (CLANG64)	8m 28s
ubuntu (clang with CUDA with OpenMP)	5m 59s
mingw (CLANG32)	7m 34s
ubuntu (clang without CUDA with OpenMP)	3m 54s
ubuntu (gcc without CUDA without OpenMP)	2m 33s
	2h 44m 36s

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