build: A few cmake cleanups and minor code rearrangements (#4359) #532

Job	Run time
windows-2019 VS2019	54m 56s
macos-12 appleclang13/C++17 py3.10 MacOS-12	24m 44s
windows-2022 VS2022	1h 5m 50s
Ubuntu latest releases gcc12 C++17 avx2 exr3.2 ocio2.3	13m 28s
macos-13 appleclang14/C++17 py3.11 MacOS-13	22m 8s
VFX2021 gcc9/C++17 py3.7 exr3.1 ocio2.0	13m 19s
Ubuntu bleeding edge gcc14 C++20 py3.12 OCIO/libtiff/exr-master avx2	22m 59s
macos-14 appleclang15/C++20 py3.11 MacOS-14-ARM	10m 33s
VFX2021 clang10/C++17 avx2 exr3.1 ocio2.0	12m 56s
Ubuntu all local builds gcc12 C++17 avx2 exr3.2 ocio2.3	13m 36s
VFX2022 gcc9/C++17 py39 exr3.1 ocio2.1	12m 4s
Ubuntu clang14 C++20 avx2 exr3.1 ocio2.1	15m 24s
VFX2022 clang13/C++17 py39 avx2 exr3.1 ocio2.1	11m 36s
Ubuntu debug gcc9/C++17, sse4.2, exr3.1	15m 36s
VFX2022 icc/C++17 py3.9 exr3.1 ocio2.1 qt5.15	21m 4s
Ubuntu static libs gcc9 C++17 exr2.4	11m 18s
VFX2022 icx/C++17 py3.9 exr3.1 ocio2.1 qt5.15	19m 23s
VFX2022 sanitizers	37m 37s
Ubuntu clang-format	3m 8s
VFX2023 gcc11/C++17 py3.10 exr3.1 ocio2.2	11m 4s
VFX2024 gcc11/C++17 py3.11 exr3.2 ocio2.3	11m 4s
VFX2021 oldest gcc9.3/C++17 py3.7 exr-3.1 no-sse no-ocio	16m 19s
VFX2021 hobbled gcc9.3/C++17 py3.7 exr-3.1 no-sse no-ocio	13m 37s
VFX2023 abi check	9m 6s
	7h 42m 49s

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