* libc/include/stdio.h (FILENAME_MAX): Changed to use __FILENAME_MAX__
if defined.
(FOPEN_MAX): Changed to use __FOPEN_MAX__ if defined.
(L_tmpnam): Changed to use __L_tmpnam__ if defined.
* libc/include/sys/config.h: Changed to set __FILENAME_MAX__
appropriately for Cygwin and RTEMS so not to exceed PATH_MAX.