Use the gstreamer mirror as the primary source and fallback to upstream if it's down.
Can't update to 2.5.19 because of https://github.com/lexxmark/winflexbison/issues/48 Fixes https://gitlab.freedesktop.org/gstreamer/gst-build/issues/43
This subproject will download and provide win32 binaries for flex and/or bison if they aren't found at configure time on Windows.