Fix a bunch of warnings, issued by GCC. #621

Job	Run time
Windows-2019 [arch x64, toolset v142, backend botan, build shared libs off, use CMake prefix path on, sanitizers off]	36m 5s
Windows-2019 [arch x64, toolset v142, backend botan, build shared libs off, use CMake prefix path off, sanitizers off]	36m 23s
Windows-2019 [arch x64, toolset v142, backend openssl, build shared libs off, use CMake prefix path on, sanitizers off]	19m 10s
Windows-2019 [arch x64, toolset v142, backend openssl, build shared libs off, use CMake prefix path off, sanitizers off]	21m 1s
Windows-2019 [arch x64, toolset ClangCL, backend botan, build shared libs off, use CMake prefix path on, sanitizers off]	39m 26s
Windows-2019 [arch x64, toolset ClangCL, backend botan, build shared libs off, use CMake prefix path off, sanitizers off]	38m 42s
Windows-2019 [arch x64, toolset ClangCL, backend openssl, build shared libs off, use CMake prefix path on, sanitizers off]	22m 54s
Windows-2019 [arch x64, toolset ClangCL, backend openssl, build shared libs off, use CMake prefix path off, sanitizers off]	22m 11s
Windows-2019 [arch x64, toolset v142, backend botan, build shared libs off, use CMake prefix path off, sanitizers on]	49m 7s
Windows-2019 [arch Win32, toolset ClangCL, backend botan, build shared libs off, use CMake prefix path on, sanitizers off]	40m 17s
Windows-2019 [arch Win32, toolset v142, backend openssl, build shared libs off, use CMake prefix path off, sanitizers off]	24m 29s
	5h 49m 45s

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