automake: Add a prefix variable to the common sources lists.

automake: Add a prefix variable to the common sources lists.

In order to do the minimal change for libdricore conversion to
automake, I need to put its Makefile.am in a subdirectory.  Automake
gets whiny/broken if you use GNU make features like "addprefix" or
"$(FILES:%=../%)" to munge your *_SOURCES.  So, use a plain old
variable to be able to substitute in that "../"

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>