stdlibc++ headers not found error on xcode 10

We have some c libraries written for our iOS project and after upgrading to Xcode 10, when compiling we get the error:

error: include path for stdlibc++ headers not found; pass ‘-std=libc++’
on the command line to use the libc++ standard library instead
[-Werror,-Wstdlibcxx-not-found]
        1 error generated.

Where do we need to make changes in out makefiles? Because I don't find the '-std='argument in them.