cmake

Add flags and functions to control real number output precision. #453

Job	Run time
linux_clang_latest	48s
fuzzing	3m 34s
linux_archs (i386)	35s
linux_gcc4	37s
linux_archs (arm32v7)	3m 34s
linux_checks (Debug, Default, Valgrind)	3m 32s
linux_gcc5_i386	48s
linux_archs (arm64v8)	3m 33s
linux_checks (Debug, Default, Sanitizer)	54s
linux_checks (Debug, ReduceBinary, Valgrind)	3m 30s
linux_archs (ppc64le)	3m 31s
linux_opts (gcc, no_fp)	42s
linux_archs (s390x)	3m 33s
linux_checks (Debug, ReduceBinary, Sanitizer)	1m 4s
linux_opts (gcc, no_reader)	4s
linux_checks (Release, Default, Valgrind)	2m 34s
linux_archs (riscv64)	3m 33s
linux_opts (gcc, no_writer)	34s
linux_checks (Release, Default, Sanitizer)	1m 18s
linux_tinycc	38s
linux_opts (gcc, no_utf8)	45s
linux_checks (Release, ReduceBinary, Valgrind)	2m 38s
linux_opts (clang, no_fp)	17s
macos_clang	16s
linux_checks (Release, ReduceBinary, Sanitizer)	2m 3s
linux_opts (clang, no_reader)	7s
windows_msvc	1m 48s
linux_opts (clang, no_writer)	16s
linux_checks (MinSizeRel, Default, Valgrind)	2m 52s
linux_opts (clang, no_utf8)	19s
codecov	24s
linux_checks (MinSizeRel, Default, Sanitizer)	1m 15s
linux_checks (MinSizeRel, ReduceBinary, Valgrind)	2m 58s
linux_gcc_latest	2m 37s
linux_checks (MinSizeRel, ReduceBinary, Sanitizer)	1m 10s
	58m 41s

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