Add Linux clang build #974

Job	Run time
Linux.3: static / ci_steps	7m 31s
Linux.2: Debug / ci_steps	7m 12s
Linux.10: clang / ci_steps	7m 23s
Linux.1: Release / ci_steps	14m 15s
Linux.4: threads=OFF / ci_steps	7m 6s
Linux.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal deflate / ci_steps	4m 7s
Linux.6: custom namespace / ci_steps	7m 2s
Linux.7: vfx2023 / ci_steps	6m 59s
Linux.8: vfx2022 / ci_steps	6m 51s
Linux.9: vfx2021 / ci_steps	6m 38s
macOS.1: Release / ci_steps	9m 33s
macOS.2: Debug / ci_steps	20m 29s
macOS.3: static / ci_steps	8m 42s
macOS.4: threads=OFF / ci_steps	7m 38s
macOS.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal imath+deflate / ci_steps	1m 18s
macOS.6: macos-13 / ci_steps	25m 2s
Windows.1: Release / ci_steps	12m 44s
Windows.2: Debug / ci_steps	39m 34s
Windows.3: static / ci_steps	13m 38s
Windows.4: threads=OFF / ci_steps	13m 16s
Windows.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal imath+deflate / ci_steps	2m 35s
Windows.6: vfx2022 / ci_steps	12m 59s
	4h 2m 32s

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