lang/cjs: unbreak build with Clang 6 (C++14 by default)

gi/boxed.cpp:552:16: error: cannot initialize return object of type 'GIFieldInfo *' (aka '_GIBaseInfoStub *') with an rvalue of type 'int'

return JS_FALSE;
       ^~~~~~~~

gi/ns.cpp:188:16: error: cannot initialize return object of type 'JSObject *' with an rvalue of type 'int'

return JS_FALSE;
       ^~~~~~~~

gi/repo.cpp:312:16: error: cannot initialize return object of type 'JSObject *' with an rvalue of type 'int'

return JS_FALSE;
       ^~~~~~~~

/usr/local/include/mozjs-24/jstypes.h:194:18: note: expanded from macro 'JS_FALSE'
#define JS_FALSE (int)0

^~~~~~

Reported by: antoine (via bug 224669)