Fix compilation on clang / OSX

Fixes #110. Compilation was failing on clang errors like this:
```
    /Users/johnboiles/cusf-standalone-predictor/pred_src/pred.c:50:44: error: expected ';' after struct
        void *options = gopt_sort(&argc, argv, gopt_start(
                                               ^
    /Users/johnboiles/cusf-standalone-predictor/pred_src/util/gopt.h:28:106: note: expanded from macro 'gopt_start'
    #define gopt_start(...)  (const void*)( const struct { int k; int f; const char *s; const char*const*l; }[]){ __VA_ARGS__, {0}}
                                                                                                             ^
```

The fix is to break out the struct definition onto a separate line.