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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.