使用cocos new HelloWorldDemo -p com.coco2dx.org -l cpp -d ~/Desktop生成的新工程，打开iOS工程，编译获得waring无数。请问，如何去除“was built for newer iOS version (7.0) than being linked (6.0)”这些warning？
xcode: 7.1.1
cocos2d-x: 3.9.1
target: 6.0
ld: warning: object file (xxxxxx-dxlotdxzjgodjjerkmarqyspkbbw/Build/Products/Debug-iphonesimulator/libcocos2d iOS.a(ftbase.ios8_x86_64.o)) was built for newer iOS version (7.0) than being linked (6.0)
ld: warning: object file (xxxxxx-dxlotdxzjgodjjerkmarqyspkbbw/Build/Products/Debug-iphonesimulator/libcocos2d iOS.a(ftinit.ios8_x86_64.o)) was built for newer iOS version (7.0) than being linked (6.0)
ld: warning: object file (xxxxxx-dxlotdxzjgodjjerkmarqyspkbbw/Build/Products/Debug-iphonesimulator/libcocos2d iOS.a(ftglyph.ios8_x86_64.o)) was built for newer iOS version (7.0) than being linked (6.0)
ld: warning: object file (xxxxxx-dxlotdxzjgodjjerkmarqyspkbbw/Build/Products/Debug-iphonesimulator/libcocos2d iOS.a(ftstroke.ios8_x86_64.o)) was built for newer iOS version (7.0) than being linked (6.0)
