Remove use of obsolete BOOST_EXPLICIT_TEMPLATE_TYPE{,_SPEC} macros.
Remove use of obsolete BOOST_EXPLICIT_TEMPLATE_NON_TYPE macro.
Remove use of obsolete BOOST_APPEND_EXPLICIT_TEMPLATE_TYPE{,_SPEC} macro.
Remove use of obsolete BOOST_APPEND_EXPLICIT_TEMPLATE_NON_TYPE macro.
Remove obsolete MSVC check from pragma guard

 git grep -h -B1 "^#\s*pragma once" | grep -v pragma | sort | uniq

is now clean.
MultiIndex: Remove obsolete MSVC version checks.
MultiIndex: Remove obsolete prevent_eti workaround.

It was previously provided for old MSVC.
MultiIndex: Remove obsolete MSVC version checks.
MultiIndex: Remove obsolete GCC version check.
MultiIndex: Remove obsolete MSVC version checks.