build: remove strings.h from header checks

We don't include strings.h anywhere.

This is also already checked for by autoconf, so us checking for it just
means a 3rd existence check during ./configure.

build_msvc/bitcoin_config.h.in

@@ -152,9 +152,6 @@
 /* Define to 1 if you have the <stdlib.h> header file. */
 #define HAVE_STDLIB_H 1
 
-/* Define to 1 if you have the <strings.h> header file. */
-#define HAVE_STRINGS_H 1
-
 /* Define to 1 if you have the <string.h> header file. */
 #define HAVE_STRING_H 1
 

configure.ac

@@ -1010,7 +1010,7 @@ if test "$TARGET_OS" = "darwin"; then
   AX_CHECK_LINK_FLAG([-Wl,-bind_at_load], [HARDENED_LDFLAGS="$HARDENED_LDFLAGS -Wl,-bind_at_load"], [], [$LDFLAG_WERROR])
 fi
 
-AC_CHECK_HEADERS([endian.h sys/endian.h byteswap.h stdio.h stdlib.h unistd.h strings.h sys/types.h sys/stat.h sys/select.h sys/prctl.h sys/sysctl.h vm/vm_param.h sys/vmmeter.h sys/resources.h])
+AC_CHECK_HEADERS([endian.h sys/endian.h byteswap.h stdio.h stdlib.h unistd.h sys/types.h sys/stat.h sys/select.h sys/prctl.h sys/sysctl.h vm/vm_param.h sys/vmmeter.h sys/resources.h])
 
 AC_CHECK_DECLS([getifaddrs, freeifaddrs],[CHECK_SOCKET],,
     [#include <sys/types.h>