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.