CMake: Support building HarfBuzz-GObject

CMake: Support building HarfBuzz-GObject

This adds support to the CMake build files to support building
HarfBuzz-GObject, which is actually a little bit complicated,
as we need to:

-Run glib-mkenums, which is a Python (for newer GLib/GObject) or PERL
 (for older, pre 2.53.4, GLib/GObject) script.  This matters more for
 Visual Studio builds as shebang lines are not supported, so we need to
 test-run it with PERL or Python to determine which is the correct
 interpretor to use.

-Next, we need to replace strings in the sources that we obtain from
 running glib-mkenums.  So, the solution here is to use a small utility
 CMake script for our purposes here, to maintain maximum compatibility.

-Ensure that things do work in the Visual Studio IDE builds.