fix the `was compiled for iOS Simulator, but linking for MacOSX` warning

We're seeing ...

```
ld: warning: ObjC object file
(/Users/fpotter/Library/Developer/Xcode/DerivedData/xctool-coaqfpnavanndzgumiqgxmfwpkjd/Build/Intermediates/otest-query.build/Debug/otest-query-ios.build/Objects-normal/i386/main.o)
was compiled for iOS Simulator, but linking for MacOSX
```

Where we used to pass `-miphoneos-version-min=7.0` in Xcode 4, we now
need to pass `-mios-simulator-version-min=7.0`.