fix ftbfs with GCC 5

  CC libgeis_util_la-geis_logging.lo
In file included from geis_bag.c:25:0:
geis_bag.c: In function '_bag_store_allocate':
geis_logging.h:56:39: error: ISO C does not support '__FUNCTION__' predefined identifier [-Werror=pedantic]
                                       __FUNCTION__, __LINE__, \
                                       ^
geis_bag.c:39:5: note: in expansion of macro 'geis_error'
     geis_error("failed to allocate bag store");
     ^
geis_bag.c: In function 'geis_bag_new':
geis_logging.h:56:39: error: ISO C does not support '__FUNCTION__' predefined identifier [-Werror=pedantic]
                                       __FUNCTION__, __LINE__, \
                                       ^
geis_bag.c:53:5: note: in expansion of macro 'geis_error'
     geis_error("failed to allocate bag");
     ^
cc1: all warnings being treated as errors
Makefile:440: recipe for target 'libgeis_util_la-geis_bag.lo' failed