Apple Mach-O Librarian error, flags invalid: -ObjC

I tried to include the Google Admob SDK in my cocos2d project, however I got the following error:

> /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool:
> -dynamic not specified the following flags are invalid: -ObjC  /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool:
> can't locate file for: -lwebp
> /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool:
> file: -lwebp is not an object file (not allowed in a library) Command
> /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool
> failed with exit code 1

error message:

Team /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool failed with exit code 1

got stuck here for a while and does anyone know what the potential problem is?

I followed the AdMob instruction and added -ObjC to the end of the linker flags.