safe-strto.c: Add safe_strtoc() for parsing complex arguments

safe-strto.c: Add safe_strtoc() for parsing complex arguments

Because people may wish to use safe-strto on systems that don't
support the more recent complex.h (from C99) or may wish to avoid
linking to libm, I've masked code related to safe_strtoc() behind
HAVE_COMPLEX.  If you define this symbol, you get safe_strtoc(), but
have to link against libm.  If you don't define this symbol, you don't
get safe_strtoc().

To turn on the symbol for this project, I've created config.h.
Usually such a config header would be created via the GNU Autotools
suite (e.g. `./configure`).