Fix executable bit check to work on Mac OS X.

### Motivation:
`find -executable` doesn't work on Mac OS X. See #671.

### Changelog:
* Patch command to work under Mac OS X. Fix #671.
* Ignore results from `virtualenv` folder.
Blast. Didn't seem to work out.

```
Test for non-executable files with execute bit set...
find: -perm: /111: illegal mode string
Done.
make: *** [style] Error 1
```