error: 'ssize_t' does not name a type (missing stddef.h)

In trying to compile a Qt c++ based application, the compiler cannot found a header file: stddef.h

Error log:
In file included from /opt/libcouchbase/include/libcouchbase/couchbase.h:29:0,
from ../../src/connector/vp_connector.cpp:8:
/opt/libcouchbase/include/libcouchbase/configuration.h:57:13: error: 'ssize_t' does not name a type
In file included from /opt/libcouchbase/include/libcouchbase/couchbase.h:31:0,
from ../../src/connector/vp_connector.cpp:8:
/opt/libcouchbase/include/libcouchbase/types.h:206:31: error: expected identifier before '*' token
/opt/libcouchbase/include/libcouchbase/types.h:210:47: error: ISO C++ forbids declaration of 'libcouchbase_ssize_t' with no type [-fpermissive]
/opt/libcouchbase/include/libcouchbase/types.h:210:47: error: 'libcouchbase_ssize_t' declared as function returning a function
......

It should be changed to unistd.h... at less in linux systems.