[PR lto/61334] Declare prototype for strnlen, if needed.

	include/
	* libiberty.h [defined (HAVE_DECL_STRNLEN) &&
	!HAVE_DECL_STRNLEN] (strnlen): New prototype.
	gcc/
	* configure.ac: Use gcc_AC_CHECK_DECLS to check for strnlen
	prototype.
	* config.in: Regenerate.
	* configure: Likewise.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@211401 138bc75d-0d04-0410-961f-82ee72b054a4