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
  that for some reason cannot work. For example, the GTK+ from MSYS2 is used
  instead of downloading one manually, but WikSparkle lacks a package in MSYS2
  and is thus downloaded.

* Some other small changes.