Build CMake with most-recent available language dialect.

When CMake is built with CMake 3.1 or later, appropriate -std=
options will be added for GNU and Clang compilers while building
C and CXX code.

This allows taking advantage of 'hidden' language features such
as move-constructors, and allows the standard library to enable
the use of more-advanced features too, where available.

This does not change CMake host compiler requirements.
Merge topic 'use-latest-language-dialects' into next

d49f1c5e Build CMake with most-recent available language dialect.