improve protobuf (#595)

packages/p/protobuf-c/xmake.lua

@@ -17,6 +17,9 @@ package("protobuf-c")
     end
 
     add_links("protobuf-c")
+    if is_plat("linux") then
+        add_syslinks("pthread")
+    end
 
     on_load(function (package)
         package:addenv("PATH", "bin")

packages/p/protobuf-cpp/xmake.lua

@@ -24,6 +24,10 @@ package("protobuf-cpp")
         add_links("protobuf")
     end
 
+    if is_plat("linux") then
+        add_syslinks("pthread")
+    end
+
     on_load(function (package)
         package:addenv("PATH", "bin")
     end)