Only provide the file descriptor array if
CONFIGURE_MAXIMUM_FILE_DESCRIPTORS > 0. If someone configured
CONFIGURE_MAXIMUM_FILE_DESCRIPTORS == 0 and the appplication uses
something which requires a file descriptor, then a linker error will
show up. An alternative would be to add a zero-length array
rtems_libio_iops[ 0 ] to librtemscpu.a which would be used in this
case.
Conditionally include some header files.