fixing `make install` (install -D) on OSX

`install -D` does not work on OSX, but since it only creates the target directories, it can easily be split out into a `mkdir -p …`.

```
install -Dpm0755 libsass.a /usr/local/lib/libsass.a
install: illegal option -- D
```
Merge pull request #387 from rodneyrehm/patch-2

fixing `make install` (install -D) on OSX