Allow parent project to override options #3148

Job	Run time
ubuntu-clang-deprecated Debug	21m 51s
ubuntu-clang-deprecated Release	22m 28s
ubuntu-clang-quaternions Debug	21m 45s
ubuntu-clang-quaternions Release	22m 38s
ubuntu-clang-tbb Debug	23m 22s
ubuntu-clang-tbb Release	24m 44s
ubuntu-clang-cayleymap Debug	21m 26s
ubuntu-clang-cayleymap Release	23m 3s
ubuntu-clang-system-libs Debug	22m 21s
ubuntu-clang-system-libs Release	22m 26s
ubuntu-no-boost Debug	21m 45s
ubuntu-no-boost Release	23m 8s
ubuntu-no-unstable Debug	18m 25s
ubuntu-no-unstable Release	18m 53s
	5h 8m 15s

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