Reworking the MSVC-related build options

[Tor Lillqvist]

Anyway, about killing the --directx-home option, even if that is done,
that does not mean having and building against the DirectX SDK (June
2010, or some earlier version) would be mandatory. It would just mean
that if the standard DXSDK_DIR environment variable does not exist,
then DirectX use is not built, i.e. like --disable-directx.

Probably the explicit --disable/enable-directx option can be removed
too; if $DXSDK_DIR exists and is valid, build against it, otherwise
not (and give a warning that the resulting build lacks functionality
and should probably not be distributed in a serious way). Why would
this need to be a configure option?

--tml