configure: Call AG_GST_PKG_CONFIG_PATH to set GST_PKG_CONFIG_PATH
configure: Call AG_GST_PKG_CONFIG_PATH to set GST_PKG_CONFIG_PATH

GST_PKG_CONFIG_PATH is used in docs/plugins directory, so
AG_GST_PKG_CONFIG_PATH must be called to set it.

https://bugzilla.gnome.org/show_bug.cgi?id=768787

pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,stfl/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good