Set ENOTSUP when PROC_PPID_ONLY is undefined

closes #452

src/pid.cpp

@@ -178,6 +178,8 @@ pid_type parent_pid(pid_type pid, error_code & ec)
 std::vector<pid_type> child_pids(pid_type pid, error_code & ec)
 {
     std::vector<pid_type> vec;
+#if defined(PROC_PPID_ONLY)
+
     vec.resize(proc_listpids(PROC_PPID_ONLY, (uint32_t)pid, nullptr, 0) / sizeof(pid_type));
     const auto sz = proc_listpids(PROC_PPID_ONLY, (uint32_t)pid, &vec[0], sizeof(pid_type) * vec.size());
     if (sz < 0)
@@ -186,6 +188,9 @@ std::vector<pid_type> child_pids(pid_type pid, error_code & ec)
         return {};
     }
     vec.resize(sz);
+#else
+    BOOST_PROCESS_V2_ASSIGN_EC(ec, ENOTSUP, system_category());
+#endif
     return vec;
 }