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

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

This required some reordering of headers to ensure that the symbol name
redefines happened before any prototypes.

v2: drop dridir now that it's unused.
v3: Consistently put spaces around += in the updated Makefile.am blocks.
v4: 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> (v2)
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>