Add #ifndef NO_PRINTF_PERCENT_N. Since we are fully standardized, we

Add #ifndef NO_PRINTF_PERCENT_N.  Since we are fully standardized, we
don't use disable %n ourselves.  But Google's Android libc is based
on our libc....  Giving them an easy knob to disable this dangerous
feature easily make their job easier without making our job any harder.
Request from Elliott @ google