Complain if -[DIU] is given an empty argument. (Like many compilers over the years, we don't accept the argument appearing in the next argv word.) Warn about this in the man page too.

x
x
x1x
x1x
xx
x12x
t16.c:10:1: Wrong number of arguments for macro c; found 1, expected 2
x1x
t16.c:11:1: Wrong number of arguments for macro c; found 0, expected 2
xx
FAILED