Fixed BUILD_NODE_EXTENSION macro redefinition

Building on Mac OSX 10.7.5 was giving the following warning:

```
../sleep.cc:2:9: warning: 'BUILDING_NODE_EXTENSION' macro redefined
#define BUILDING_NODE_EXTENSION
        ^
<command line>:4:9: note: previous definition is here
#define BUILDING_NODE_EXTENSION 1
        ^
1 warning generated.
```

This change fixes it.