Document how to use libstdc++ from GCC-6.4.0 or newer

[Paul Hargrove]

We document that with Intel, Clang, and hopefully soon PGI, one must be using the libstdc++ from GCC 6.4.0 or newer. However, we currently provide not one shred of direct documentation on how one is to accomplish this (one cannot expect users to find my comments in the issue tracker).

At the 2019-07-24 Pagoda issue meeting I accepted an Action Item to provide suitable documentation in time for the Sep 2019 release.

[Paul Hargrove]

Document libstdc++ query/control for non-GNU CXX

This commit adds a new file documenting how to query and control the libstdc++ to be used by the Clang, Intel and PGI C++ compilers on Linux. It additionally provides an appropriate cross-reference from README.md, grouping it with a relocated reference to local-gcc.md.

Resolves issue #240

→ <<cset c85b7c65adf2>>