Fix bug building libyapi.dylib on OS X

To build from scratch on Mac OS X I manually removed all .o, .a and .dylib files then ran 'make'.  The build stopped at the final linking step with:
  make: *** No rule to make target `osx/yapi/libyapi.dylib', needed by `osx'.  Stop.

My proposed change fixes this bug and allow the make to complete