Fix building without GStreamer 0.10

Previously, you could build with GStreamer 1.0, but the 0.10 devel files
still had to be installed. Otherwise, the AM_GST_PLUGIN_CHECK macro was
flagged as unknown, and Automake would complain because it started with
AM_. Marking it as "allowed" prevents an error when the macro is not
defined anywhere.
Upgrade to newer version of gi_blorb

Zarf released a small bugfix to gi_blorb.c quite a while ago.
Fix wraparound bug

Better compiler warnings turned up this bug that thankfully never
manifested itself: a calculation using unsigned ints that might have
wrapped around. It was subsequently checked if it had gone below 0, but
of course that is impossible with unsigned ints. Now we check beforehand
if the calculation would wrap around.
Use gdk_color_to_string() to name colors

Can cut out some custom code by relying on gdk_color_to_string().