config: remove explicit undef of unset values

config: remove explicit undef of unset values

Rather than explicitly clearing any setting of undefined values in our
rte_config.h file, it's better to instead just add a comment that the
value is not set. Using a comment allows the user to set the value using
CFLAGS or similar mechanism without the config file clearing the value
again.

The text used "<VALUE> is not set" is modelled after the kernel approach
of doing the same thing.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Morten Brørup <mb@smartsharesystems.com>
Acked-by: Tyler Retzlaff <roretzla@linux.microsoft.com>
Signed-off-by: David Marchand <david.marchand@redhat.com>