Cannot build tests on Mac OS X
Trying to follow the procedure _Running nimx samples_ to build tests, I get an error from XCode:

```
MacBook-Pro-de-Peter:test peter$ /Users/peter/.nimble/bin/nake 
No task specified, running default task defined by nakefile.
Build settings from command line:
    SDKROOT = macosx10.13
    SYMROOT = build

xcodebuild: error: 'Xcode/SDL/SDL.xcodeproj' does not exist.
MacBook-Pro-de-Peter:test peter$ pwd
/Users/peter/.nimble/pkgs/nimx-0.1/test
```
 