Fix compile error [-Werror=unused-value] in atomic_store_voidptr().

dev_22 fails to compile on Linux platform when built with -DCMAKE_BUILD_TYPE=Maintainer with following failure : 

In file included from src/Cache_inode/cache_inode_readdir.c:42:0:
src/include/abstract_atomic.h: In function ‘atomic_store_voidptr’:
src/include/abstract_atomic.h:2179:30: error: value computed is not used [-Werror=unused-value]
cc1: all warnings being treated as errors
make[2]: *** [Cache_inode/CMakeFiles/cache_inode.dir/cache_inode_readdir.c.o] Error 1