GDB: Fix detection of ELF support when configuring with -Werror=implicit-function-declaration

I am getting

    I'm sorry, Dave, I can't do that.  Symbol format `elf64-littleriscv' unknown.

errors after updating from GDB 8.3 to 10.  Bisecting showed that since
commit 1ff6de0312 ("bfd, ld: add CTF section linking"), bfd.h depends
on strncmp() being present, so configuring with
-Werror=implicit-function-declaration results in the check for ELF
support in BFD failing:

    .../gdb/gdb/../bfd/elf-bfd.h: In function 'bfd_section_is_ctf':
    .../gdb/gdb/../bfd/elf-bfd.h:3086:10: error: implicit declaration of function 'strncmp' [-Werror=implicit-function-declaration]
       return strncmp (name, ".ctf", 4) == 0 && (name[4] == 0 || name[4] == '.');

gdb/ChangeLog:

	* acincludde.m4 (GDB_AC_CHECK_BFD): Include string.h in the test
	program.

Change-Id: Iec5e21d454c2a544c44d65e23cfde552c424c18e

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2020-11-28  Alex Richardson  <Alexander.Richardson@cl.cam.ac.uk>
+
+	* acincludde.m4 (GDB_AC_CHECK_BFD): Include string.h in the test
+	program.
+
 2020-11-27  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* printcmd.c (skip_over_slash_fmt): Reorder code to ensure in_fmt

gdb/acinclude.m4

@@ -266,6 +266,7 @@ AC_DEFUN([GDB_AC_CHECK_BFD], [
     [AC_LINK_IFELSE(
        [AC_LANG_PROGRAM(
 	  [#include <stdlib.h>
+	   #include <string.h>
 	   #include "bfd.h"
 	   #include "$4"],
 	  [return $3;]

gdb/configure

@@ -17236,6 +17236,7 @@ else
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 #include <stdlib.h>
+	   #include <string.h>
 	   #include "bfd.h"
 	   #include "elf-bfd.h"
 int
@@ -17349,6 +17350,7 @@ else
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 #include <stdlib.h>
+	   #include <string.h>
 	   #include "bfd.h"
 	   #include "mach-o.h"
 int