zmqpp: fix build with or1k and gcc < 6

Use CONFIG variable to disable optimizations when or1k and gcc < 6 are
detected otherwise set CONFIG to release or debug depending on
BR2_ENABLE_DEBUG

Fixes:
 - http://autobuild.buildroot.net/results/523e58eefba7ef23a09ef53160da22190ccbb098



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>