Fixes https://github.com/behdad/harfbuzz/issues/576

Clarify the last step of release

builds: Support HarfBuzz-GObject and introspection in CMake builds

hb-ot-shape.cc:287:18: error: comparison of constant 32 with expression of type 'hb_unicode_general_category_t' is always true [-Werror,-Wtautological-constant-out-of-range-compare]

No flag is safe, over time... See previous commit.

The commit f559c633 caused
"undefined-shift" errors in Myanmar shaper as we changed the
numeric value of type D from 19 to 32 there, making the
"FLAG_SAFE (info.myanmar_category())" wrong.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3546

This adds support for introspection builds on Windows that is enabled by
the HB_HAVE_INTROSPECTION option, which will also enable HB_HAVE_GOBJECT
(and so HB_HAVE_GLIB) as they are required for introspection.

In order for this to work one must ensure that the Python installation
listed in PYTHON_EXECUTABLE is the same Python release series that was
used to build _giscanner.pyd (the Python module that is used by
g-ir-scanner), with the same architecture.  PKG_CONFIG_PATH and PATH
must be set correctly if $(PREFIX)\bin and/or $(PREFIX)\lib\pkgconfig
are not in the standard PATH and PKG_CONFIG_PATH, which is actually
in-line with the *NIX builds.

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.

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.

The new unorm2_normalize has been public API since ICU 4.4, while
the old unorm_normalize has been deprecated since ICU 56.

Now, beginning with ICU 60, unorm_normalize will also be annotated
U_DEPRECATED and trigger the compiler -Wdeprecated-declarations warning.

c:\projects\harfbuzz\src\hb-set-private.hh(151): error C2327: 'hb_set_t::page_t::v': is not a type name, static, or enumerator [C:\projects\harfbuzz\build\harfbuzz.vcxproj]

error: static_assert expression is not an integral constant expression

Fixes clang "non-const reference cannot bind to vector element" error.

Fixes https://github.com/behdad/harfbuzz/pull/23