Handle the standard windows-3rd-party setup (used by fgmeta) with no
extra options, and also handle the slightly odd setup we use on Jenkins.
Try to tolerate all permutations of setting MSVC_3RDPARTY_ROOT to
different places in the hierarchy.
We no longer try to guess Boost_INCLUDEDIR by looking at parent dirs of
MSVC_3RDPARTY_ROOT, since this seemed kind of bad to me. Let’s try
it and see.
Check for the 3rdparty dir in the parent of build dir, not the parent of the source dir. For the recommended build layout, this is the same location, but for super-builds using fgmeta it's not (and the source tree should not be touched).
If this causes anyone issues, please let me know, since it's possible more flexibility is needed to set the path explicitly.
