scripts: make git-update.sh build with all cores available
scripts: make git-update.sh build with all cores available

The git-update.sh now builds with all cores available. In case of
failure it defaults to 1

The developer can still override this by setting -j to something else
in MAKEFLAGS, as stated by 299605dfe2f97fca330161ff01a392e1a85fe422.

https://bugzilla.gnome.org/show_bug.cgi?id=766666

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer