Check for strnlen declaration to fix Solaris 10 build

[Nick Clifton]

Hi Rainer,

> and in a couple of other places.  The platform lacks strnlen, and while
> libiberty.h can provide a fallback declaration, the necessary configure
> test isn't run.
> 
> Fixed with the following patch.  Tested on i386-pc-solaris2.10.
> 
> Ok for master and the 2.37 branch?

Approved for both - please apply.

Cheers
   Nick