package/zeromq: enable kernel-based feature flags

The current configuration system does not check for cached variables for
these flags, and thus they are always disabled when cross-compiling.
This patch fixes the configuration system to use cached variables and
enables them at configuration time.

Signed-off-by: Lionel Orry <lionel.orry@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>