sox: adding optional lame dependency

i suffered the following:

    $ sox foobar.mp3 -C12 test.mp3
    sox FAIL formats: can't open output file `test.mp3': SoX was compiled with neither MP2 nor MP3 encoding support

after compiling with lame encoding mp3 went like a charm.
sox configure log:

```
mp2/mp3....................yes
 id3tag....................no
 lame......................yes
 lame id3tag...............yes
 dlopen lame...............no
 mad.......................yes
 dlopen mad................no
 twolame...................no
```

> admittedly, `brew install`ing lame first should work as well but this is a nice convenience and should keep people from [getting confused](http://superuser.com/questions/246918/why-isnt-sox-able-to-convert-to-mp3).

credit goes to [gregorio garcia karman](http://ggkarman.de/tech/building-sox-with-mp3-support-on-osx/).