OSX: fix compatibility with BSD `mkdir`.

`--parent` flag does not exist in the BSD mkdir in Mac OS X. Short `-p` is there, however.

```
$ make firefox
mkdir: illegal option -- -
usage: mkdir [-pv] [-m mode] directory ...
make: *** [firefox] Error 64
```
Merge pull request #420 from mtigas/master

OSX: fix compatibility with BSD `mkdir`.