OSX: fix compatibility with BSD `mkdir`.

Short & sweet:

`--parent` flag does not exist in the [BSD mkdir](https://www.freebsd.org/cgi/man.cgi?query=mkdir&sektion=1) in Mac OS X — at least, the one that came with my Mac OS X 10.8.4. (Looks like it’s specific to [GNU mkdir](https://www.gnu.org/software/coreutils/manual/html_node/mkdir-invocation.html).)

Short `-p` is there, however.

Fixes this, basically:
```
$ make firefox
mkdir: illegal option -- -
usage: mkdir [-pv] [-m mode] directory ...
make: *** [firefox] Error 64
```