Add into the NMake Makefiles to build the DirectWrite shaping backend,
but as PR #134 mentions, this is considered to be in an experimental state,
so don't include this in the build by default for now. This is most probably
going to replace the Uniscribe backend eventually, since DirectWrite is meant
to be Uniscribe's replacement, and is needed for Windows Store apps if a
system shaping API is to be used.
This adds a pre-configured config.h template that can be used for Visual
Studio builds, where autotools is not normally available. This has the
configs that are suitable for Visual Studio builds, as well as all the
features used for Windows builds enabled (HAVE_OT, HAVE_FALLBACK and
HAVE_UNISCRIBE).
Note that the optional features are not enabled here, they are enabled by
/D's (or -D's) in the NMake Makefiles as requested.
