gpsd: work-around ICE when compiling for microblaze with GCC5

GCC 5.X generates an ICE when compiling gpsd for microblaze due
to optimizations, so lets disable it for now to works around
this problem.

A bug was reported to the gcc bug tracker:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68485



Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>