Silence a bunch of format-zero-length warnings

Silence a bunch of format-zero-length warnings

This is in gcc 4.9.0:

  wt-status.c: In function ‘wt_status_print_unmerged_header’:
  wt-status.c:191:2: warning: zero-length gnu_printf format string [-Wformat-zero-length]
    status_printf_ln(s, c, "");
    ^

We could pass -Wno-format-zero-length, but it seems compiler-specific
flags are frowned upon, so let's just avoid the warning altogether.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>