cmake

Add flags and functions to control real number output precision. #457

Job	Run time
linux_clang_latest	2m 37s
linux_archs (i386)	1m 25s
linux_archs (arm32v7)	12m 10s
linux_archs (arm64v8)	13m 58s
linux_gcc4	53s
linux_archs (ppc64le)	10m 56s
linux_gcc_latest	3m 1s
linux_archs (s390x)	11m 32s
linux_checks (Debug, Default, Valgrind)	9m 19s
linux_archs (riscv64)	13m 23s
linux_checks (Debug, Default, Sanitizer)	1m 15s
linux_gcc5_i386	1m 10s
linux_checks (Debug, ReduceBinary, Valgrind)	8m 1s
linux_opts (gcc, no_fp)	41s
linux_checks (Debug, ReduceBinary, Sanitizer)	1m 2s
linux_tinycc	35s
linux_opts (gcc, no_reader)	31s
linux_checks (Release, Default, Valgrind)	2m 34s
macos_clang	37s
linux_opts (gcc, no_writer)	33s
linux_checks (Release, Default, Sanitizer)	1m 35s
linux_opts (gcc, no_utf8)	47s
linux_opts (clang, no_fp)	44s
codecov	1m 7s
linux_opts (clang, no_reader)	26s
linux_checks (Release, ReduceBinary, Valgrind)	6m 43s
linux_opts (clang, no_writer)	33s
linux_checks (Release, ReduceBinary, Sanitizer)	1m 12s
linux_opts (clang, no_utf8)	43s
fuzzing	10m 23s
linux_checks (MinSizeRel, Default, Valgrind)	2m 56s
linux_checks (MinSizeRel, Default, Sanitizer)	1m 38s
windows_msvc	1m 25s
linux_checks (MinSizeRel, ReduceBinary, Valgrind)	6m 37s
linux_checks (MinSizeRel, ReduceBinary, Sanitizer)	1m 12s
	2h 14m 14s

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