This adds to the autotools build system so that the (experimental)
DirectWrite support for HarfBuzz is built (and dist'ed).

This moves all the source listings in src/Makefile.am,
src/hb-ucdn/Makefile.am and util/Makefile.am into separate Makefile
snippets, so that they may be shared between different Makefile-based
build systems, such as NMake for Visual Studio.