Restore (some) compiler sanity

-fno-strict-overflow tells gcc and clang to handle signed integer and
(at least on gcc) pointer arithmetic wraparound using twos-complement
representation like deity intended.

-fno-delete-null-pointer-checks tells gcc not to "optimize" away
programmer added safeguards. Really.

Suggested by Demi Marie Obenour.

configure.ac

@@ -37,7 +37,7 @@ fi
 AS=${AS-as}
 AC_SUBST(AS)
 if test "$GCC" = yes; then
-    cflags_to_try="-fno-strict-aliasing -Wempty-body"
+    cflags_to_try="-fno-strict-aliasing -fno-strict-overflow -fno-delete-null-pointer-checks -Wempty-body"
     AC_MSG_CHECKING([supported compiler flags])
     old_cflags=$CFLAGS
     echo