Add Boost location functionality to mira-client SCons

The current mira-client SCons build system (which consists of the top-level /mira-client/SConstruct script and the /mira-client/src/SConscript script) is incapable of locating Boost and discovering its version. This means that the version of Boost with which Mira Client is to be built has to be specified manually.

This blueprint outlines how we can go about fixing this problem. The SCons Boost Tool created for the RealityForge project and released under a GPL v2 license is available here: https://realityforge.vrsource.org/viewcvs/trunk/scons-addons/src/SConsAddons/Options/Boost.py?root=scons-addons&view=markup We can implement this Tool into the mira-client SCons system to locate Boost for us, check that the installed version of Boost is greater than or equal to a pre-defined required version and automatically provide the path and version of Boost to the build system. This will make the build system far more dynamic.