[Mesa-dev] Build with optimization + debug and 'Mesa errors' messages

[Fabio Pedretti]

After commit 122345876 there is no way to change compiler optimization flag on 
a debug build, since -O0 gets added after your CFLAGS. Can this be changed?

Also it would be nice to always enable, also on non debug builds, the messages 
about Mesa errors in output_if_debug. It can be enabled with MESA_DEBUG env 
var, but having it enabled by default could help to detect possible problems, 
it also won't spam too much the console since it can detect and report 
duplicate messages without printing it every time.