Remove obsolete MSVC check from pragma guard

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

is now clean.

[SVN r85952]
xpressive: Remove obsolete GCC version check.

[SVN r86123]
Remove BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION

Process #ifdef...#endif blocks.

[SVN r86243]
docs for stuff in regex_actions.hpp, refs #7662

[SVN r86267]