Check for strnlen declaration to fix Solaris 10 build

Check for strnlen declaration to fix Solaris 10 build

binutils currently fails to compile on Solaris 10:

/vol/src/gnu/binutils/hg/binutils-2.37-branch/git/bfd/opncls.c: In function 'bfd_get_debug_link_info_1':
/vol/src/gnu/binutils/hg/binutils-2.37-branch/git/bfd/opncls.c:1231:16: error: implicit declaration of function 'strnlen' [-Werror=implicit-function-declaration]
 1231 |	  crc_offset = strnlen (name, size) + 1;
      |		       ^~~~~~~
/vol/src/gnu/binutils/hg/binutils-2.37-branch/git/bfd/opncls.c:1231:16: error: incompatible implicit declaration of built-in function 'strnlen' [-Werror]
/vol/src/gnu/binutils/hg/binutils-2.37-branch/git/bfd/opncls.c: In function 'bfd_get_alt_debug_link_info':
/vol/src/gnu/binutils/hg/binutils-2.37-branch/git/bfd/opncls.c:1319:20: error: incompatible implicit declaration of built-in function 'strnlen' [-Werror]
 1319 |	  buildid_offset = strnlen (name, size) + 1;
      |			   ^~~~~~~

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.

2021-07-06  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	bfd:
	* configure.ac: Check for strnlen declaration.
	* configure, config.in: Regenerate.

	binutils:
	* configure.ac: Check for strnlen declaration.
	* configure, config.in: Regenerate.