Reworking the MSVC-related build options

[Tor Lillqvist]

> * --enable-/--disable-directx should really force building with/without directx.
>   default enabled is probably the best choice).
> * only if it's enabled, check for the dxsdk. using an environment
>   variable instead of separate flag is fine, IMHO, as long as
>   that variable is properly documented in ./configure --help output.
> * when enabled, but no proper dxsdk found, ./configure fails w/ a
>   proper error message

Exactly my opinion, too. I don't think anybody is disagreeing much
here. What I want is to kill the --with-directx-home option, not the
--enable/disable-directx one.

--tml