Include the fallback sources in the build, and update
src/Makefile.sources so that it can be read by the CMake build files.
Fix a typo in the DirectWrite configure option so that we can properly
enable DirectWrite builds.
Also, when building the utility program, install them as well.
Instead of searching for freetype using pkg-config, use the FindFreetype
feature of CMake. This allows for better integration with other projects
that make use of CMake.
Fixes https://github.com/behdad/harfbuzz/issues/518
