Patch to add ACONCAT macro to libiberty

[DJ Delorie]

> + extern unsigned long concatlen PARAMS ((const char *, ...));
> + extern char *concatcpy PARAMS ((char *, const char *, ...));
> + extern char *concatcpy2 PARAMS ((const char *, ...));
> 
> I thought these might be useful in a general sense, so I made their
> names friendly and documented them in libiberty.h.

I (slightly) prefer something like concat_copy or concat_length rather
than a random abbreviation.