[WIP] Build2 port

[springmeyer]

This is an experiment to use build2 for hpp-skel.

Next step is to upgrade to build2 0.8.0 with supports header-only libraries:

 * Support for 'binless' (binary-less aka header-only) libraries.

   A header-only library (or, in the future, a module interface-only library)
   is not a different kind of library compared to static/shared libraries but
   is rather a binary-less, or binless for short, static or shared library.
   Whether a library is binless is determined dynamically and automatically
   based on the absence of source file prerequisites. See the build system
   manual for details.

from https://lists.build2.org/archives/announce/2018/000010.html