swrast: Build the driver into the shared mesa_dri_drivers.so.

swrast: Build the driver into the shared mesa_dri_drivers.so.

v2: drop dridir now that it's unused.
v3: Fix linking after rebase when building just swrast from classic but a
    drm-using gallium driver.
v4: Consistently put spaces around += in the updated Makefile.am block.
v5: Set a global driverAPI variable so loaders don't have to update to
    createNewScreen2() (though they may want to for thread safety).

Reviewed-by: Matt Turner <mattst88@gmail.com> (v3)
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>