More progress for the pidgin++ package.

* Use the new --pkgbuild option from winbuild script.
  This will handle disabling of binary downloads so that a purely source-based
  package can be created. There is currently one remaining binary dependency,
  WinSparkle.

* Fix customization of the GTK+ runtime.
  This is done when creating the GTK+ runtime from Fedora servers and is now
  replicated into the PKGBUILD so that the MSYS2 GTK+ can also have the
  customizations applied.

* Other minor changes.