Fix required CMake version

[Flamefire]

cxx_std_11 requires CMake 3.8

[pdimov]

There aren't any mentions of cxx_std_11 in CMakeLists.txt though.

[Flamefire]

IIRC I did it because some dependency does which "bubbled up" here and caused errors. Seen in failures of the workflow testing CMake versions

It doesn't matter much any more as we declared CMake 3.8 as the baseline for Boost: https://github.com/boostorg/boost/blob/7331414cbb8a1d645754a5229fd02f68da9d0103/CMakeLists.txt#L5

[Flamefire]

@pdimov: Can you take a look at this:

serialization/CMakeLists.txt

Lines 116 to 118 in 097a6c6

	if(BOOST_SUPERPROJECT_VERSION AND NOT CMAKE_VERSION VERSION_LESS 3.13)
	boost_install(TARGETS boost_serialization boost_wserialization VERSION ${BOOST_SUPERPROJECT_VERSION} HEADER_DIRECTORY include)
	endif()

It looks like this would do the installation twice on CMake >= 3.13