Make clang asan work with FORTIFIED_SOURCE again.

Short version: clang asan hates the glibc strcmp macro in
bits/string2.h if you are passing it a constant string argument of
length two or less.  (I could be off by one here, but that's the
basic idea.)

Closes issue 14821.

changes/bug14821

@@ -0,0 +1,4 @@
+  o Major bugfixes (compilation):
+    - Correctly repair hardened builds under the clang compiler. Previously,
+      our use of _FORTIFY_SOURCE would conflict with clang's address
+      sanitizer. Closes ticket 14821.

src/common/compat.h

@@ -42,6 +42,15 @@
 #include <netinet6/in6.h>
 #endif
 
+#if defined(__has_feature)
+#  if __has_feature(address_sanitizer)
+/* Some of the fancy glibc strcmp() macros include references to memory that
+ * clang rejects because it is off the end of a less-than-3. Clang hates this,
+ * even though those references never actually happen. */
+#    undef strcmp
+#  endif
+#endif
+
 #include <stdio.h>
 #include <errno.h>