Fix post-install.py on Windows without pkg-config

Aleksandr Mezinrequested to merge
amezin/gtk:fix-postinstall-no-pkgconfig into master

  1. Prepend gtk_bindir to PATH. If GLib is built as a subproject, this will allow running glib-compile-schemas and gio-querymodules without full path.

  2. Don't fail when pkg-config is not available. Just try to run everything without full paths (as if pkg-config variables were unavailable).

Also use argparse to parse command line arguments.

Fixes #3723 (closed)

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