configure: Add rpath to prepare-builtins util

v2: use space instead of '=' to make Mac happy

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Jeroen Ketema <j.ketema@imperial.ac.uk>
Implement generic mad_sat

v2: Fix trailing whitespace
    Fix signed long overflow
    improve comment

v3: fix typo

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Tom Stellard <tom@stellard.net>