Dont try to build a real static binary on Mac OS X/Darwin

I dont remember the exact details, but AFAIR this is not supported on Darwin. This fixes building on Mac OS X/Darwin.

I build SassC using 

```
SASS_LIBSASS_PATH=../libsass/ make
```

My directory layout is:

```
sassc
 + libsassc 
 + sassc
```
And i am in the sassc/sassc directory when executing the command. This works for me on linux and mac.