Add support to USE_LANGUAGES for C++ standards from c++0x to gnu++14.

This allows packages to specify the version of the standard that they
require, and the infrastructure then distils that down in a similar way
to GCC_REQD to the newest standard, avoiding clashes with different -std
requirements based on CXXFLAGS.

Broad concensus on tech-pkg and tested in bulk builds.

(jperkin)