First build of the Pidgin++ package.

* Add some makedepends.

* Use a Bazaar branch targeted at building with makepkg.

* Use the winbuild.sh script instead of direct calls to make.

  The winbuild and pidgin-windev scripts have been updated to allow creating a
  minimal environment only with the dependencies that are missing in MSYS2 or for
  some reason cannot work.

  For example, the GTK+ from MSYS2 is used instead of downloading one manually,
  but WikSparkle does not have a package in MSYS2 and is thus downloaded by
  --prepare-msys2.

* Some other small changes.