bash-completion: Respect the prefix
bash-completion: Respect the prefix

Don't try and install the bash helpers outside the defined prefix.

https://bugzilla.gnome.org/show_bug.cgi?id=744877

mparis/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,jpxiong/gstreamer,mparis/gstreamer,jpxiong/gstreamer,centricular/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,centricular/gstreamer,mparis/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer