Refactor the linux-dri config files so that linux-dri-x86 inherits from

Refactor the linux-dri config files so that linux-dri-x86 inherits from
linux-dri.  Turn on -O by default (-O2 provides a slight improvement in
performance, at a large cost to debuggability).  Turn off -fPIC on x86 by
default.  Turn on -Wall, to catch more stupid mistakes.  This could be both done
cleaner, and done for more config files, but it works for me for now.