gnu: Add a C++14 variant of Boost for packages that need it.

Fixes <https://bugs.gnu.org/33605> and <https://bugs.gnu.org/33610>.

* gnu/packages/boost.scm (boost-cxx14): New variable.
* gnu/packages/compression.scm (innoextract)[inputs]: Use it.
* gnu/packages/mpd.scm (ncmpcpp)[inputs]: Use it.