Log Message:
boost & boost-libs: bump required GCC to 6, and state use of c++14.

This gets the boost-libs package building on NetBSD/i386 8.0 again,
the in-tree gcc 5.5 is apparently now too old.

The c++14 bump comes from the warning emitted during build while
trying to use earlier c++ standards:

  The minimum language standard to use Boost.Math will be C++14
  starting in July 2023 (Boost 1.82 release)

This despite this actually being the 1.82 release, it's still "just"
a warning, but this gets rid of the warning, and the build succeeds.