Relax base64 decoding: allow spaces and tabs between the crc and footer. #648

Job	Run time
Windows-2019 [arch x64, toolset v142, backend botan, build shared libs off, use CMake prefix path on, sanitizers off]	38m 1s
Windows-2019 [arch x64, toolset v142, backend botan, build shared libs off, use CMake prefix path off, sanitizers off]	35m 58s
Windows-2019 [arch x64, toolset v142, backend openssl, build shared libs off, use CMake prefix path on, sanitizers off]	20m 13s
Windows-2019 [arch x64, toolset v142, backend openssl, build shared libs off, use CMake prefix path off, sanitizers off]	26m 15s
Windows-2019 [arch x64, toolset ClangCL, backend botan, build shared libs off, use CMake prefix path on, sanitizers off]	36m 48s
Windows-2019 [arch x64, toolset ClangCL, backend botan, build shared libs off, use CMake prefix path off, sanitizers off]	38m 12s
Windows-2019 [arch x64, toolset ClangCL, backend openssl, build shared libs off, use CMake prefix path on, sanitizers off]	24m 22s
Windows-2019 [arch x64, toolset ClangCL, backend openssl, build shared libs off, use CMake prefix path off, sanitizers off]	28m 16s
Windows-2019 [arch x64, toolset v142, backend botan, build shared libs off, use CMake prefix path off, sanitizers on]	52m 23s
Windows-2019 [arch Win32, toolset ClangCL, backend botan, build shared libs off, use CMake prefix path on, sanitizers off]	44m 14s
Windows-2019 [arch Win32, toolset v142, backend openssl, build shared libs off, use CMake prefix path off, sanitizers off]	26m 9s
	6h 10m 51s

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