[2.26 COMMITTED] Fix string/bug-strncat1.c build with GCC 8.

[fweimer@redhat.com (Florian Weimer)]

From: Joseph Myers <joseph@codesourcery.com>

GCC 8 warns about strncat calls with truncated output.
string/bug-strncat1.c tests such a call; this patch disables the
warning for it.

Tested (compilation) with GCC 8 for x86_64-linux-gnu with
build-many-glibcs.py (in conjunction with Martin's patch to allow
glibc to build).

	* string/bug-strncat1.c: Include <libc-diag.h>.
	(main): Disable -Wstringop-truncation for strncat call for GCC 8.

(cherry picked from commit ec72135e5f1d061cb5cf7cd1b855fd6290be10d9)

2017-11-14  Joseph Myers  <joseph@codesourcery.com>

	* string/bug-strncat1.c: Include <libc-diag.h>.
	(main): Disable -Wstringop-truncation for strncat call for GCC 8.

diff --git a/string/bug-strncat1.c b/string/bug-strncat1.c
index f1b5c37c5c..b22beba4d3 100644
--- a/string/bug-strncat1.c
+++ b/string/bug-strncat1.c
@@ -4,13 +4,21 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <libc-diag.h>
 
 char d[3] = "\0\1\2";
 
 int
 main (void)
 {
+  DIAG_PUSH_NEEDS_COMMENT;
+#if __GNUC_PREREQ (8, 0)
+  /* GCC 8 warns about strncat truncating output; this is deliberately
+     tested here.  */
+  DIAG_IGNORE_NEEDS_COMMENT (8, "-Wstringop-truncation");
+#endif
   strncat (d, "\5\6", 1);
+  DIAG_POP_NEEDS_COMMENT;
   if (d[0] != '\5')
     {
       puts ("d[0] != '\\5'");