changes to bsd.lib.mk can handle building it early enough. Don't
use the same rule for ss_err.h and ss_err.c, else `make -jN' would
run the rule twice concurrently. Don't put ss_err.c out of order
in SRCS; doing so was a kludge to get ss_err.h built early enough
for plain `make'.
Don't put a non-generated file in CLEANFILES.