Partially revert bba9c28 "configure: use LIB_EXT rather than hardcoded .so"

Partially revert bba9c28 "configure: use LIB_EXT rather than hardcoded .so"

Filenames passed to dlopen() don't need to use the platform's default extension
for shared libraries.

Using the '.so' extension when dlopen()ing DRI drivers is hardcoded into mesa
and the X server, so it should be hardcoded here in the Makefile as well.

A similar fix is probably also needed for gallium DRI drivers.

(Consider that if we were starting from scratch, perhaps we would use a custom
extension like .dri instead)

Cc: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>