Minor change to regex.c

[law@redhat.com]

> This patch fixed the problem for me on Solaris2.7.  The macro checks
> are similar to what has long worked in libiberty's symcat.h.
> 
> Okay to install?
> 
> 
> 
> 2001-11-15  Kaveh R. Ghazi  <ghazi@caip.rutgers.edu>
> 
> 	* regex.c: Check defined(__STDC__) || defined(HAVE_STRINGIZE)
> 	to determine whether cpp ISO C token pasting is available.
OK.  Thanks for taking care of this.

jeff