Improve Configure detection of memmem() [perl #133760].

Linux systems have memmem, but the header prototype is only visible if
the C library, but didn't check if the correct prototype is available.
This patch compiles & runs a test program that will fail if the prototype
is needed but not available.

This does not completely close [perl #133760].  The tests for strlcat()
and strlcpy() may also need to be similarly changed.  Also, this patch
does not change whether _GNU_SOURCE is defined or not.  Presumably that
would be done separately in the linux hints file.

diff --git a/Configure b/Configure
index 6041b0b..a83cbdf 100755 (executable)
--- a/Configure
+++ b/Configure
@@ -16155,8 +16155,47 @@ set mbtowc d_mbtowc
 eval $inlibc
 
 : see if memmem exists
-set memmem d_memmem
-eval $inlibc
+echo " "
+echo "Checking if you have a working memmem()" >&4
+$cat >try.c <<EOCP
+#$d_gnulibc HAS_GNULIBC         /**/
+#if defined(HAS_GNULIBC) && !defined(_GNU_SOURCE)
+#   define _GNU_SOURCE
+#endif
+#include <stdio.h>
+#include <stdlib.h>
+#include <stddef.h>
+#include <string.h>
+int main(int argc, char **argv)
+{
+    char *big = "abcdefghiabcdefghi";
+    char *little = "def";
+    char *rtn;
+    ptrdiff_t diff;
+    rtn = (char *) memmem(big, strlen(big), little, strlen(little));
+    diff = rtn - big;
+    exit(diff == 3 ?  EXIT_SUCCESS : EXIT_FAILURE);
+}
+EOCP
+set try
+if eval $compile; then
+    `$run ./try`
+    rc=$?
+    case "$rc" in
+        0)  echo "Yes, you do." >&4
+            val="$define"
+            ;;
+        *)  echo "Well, you have memmem, but it isn't working." >&4
+            val="$undef"
+            ;;
+    esac
+else
+    echo "No, you do not." >&4
+    val="$undef"
+fi
+set d_memmem
+eval $setvar
+$rm_try
 
 : see if memrchr exists
 set memrchr d_memrchr