Reworking the MSVC-related build options

[Tor Lillqvist]

>>>> --directx-home

>>> Well, you can find it in registry, but there is no standard variable that
>>> points to it.

>> But oowintool manages to find it, s

> oowintool provides nothing to this regard currently

Ah you are right. But configure.ac by default takes it from the
environment, it claims "A standard installation of the DirectX SDK
sets $DXSDK_DIR". Is that not true then? You don't have that
environment variable? It has always worked for me, I have never
specified --directx-home...

--tml