Linux: enable building with a local version of libssl.

net/third_party/nss/ssl/bodge/secure_memcmp.c

+// This file exists to provide the secure memcmp function. This was added in
+// NSS 3.12.5.
+
+#include <stdlib.h>
+
+/*
+ * Perform a constant-time compare of two memory regions. The return value is
+ * 0 if the memory regions are equal and non-zero otherwise.
+ */
+int
+NSS_SecureMemcmp(const void *ia, const void *ib, size_t n)
+{
+    const unsigned char *a = (const unsigned char*) ia;
+    const unsigned char *b = (const unsigned char*) ib;
+    size_t i;
+    unsigned char r = 0;
+
+    for (i = 0; i < n; ++i) {
+        r |= *a++ ^ *b++;
+    }
+
+    return r;
+}