Fix for broken Makefile on OS X with homebrew

[josegonzalez]

From logs:

so idk enough about bash to make the pr myself
but I have a small change to the grok makefile to make it work on os x
or at least with homebrew

    if command -v brew >/dev/null; then
    PREFIX?=$(brew --prefix)
    end

right around here: https://github.com/jordansissel/grok/blob/master/Makefile
That will make grok compile correctly under homebrew, assuming you have libevent and tokyo cabinet installed. I tried searching for directions yesterday, but failed to find how to make multiple if statements

[ryandesign]

I would encourage the developer of grok to decline to make any such package-manger-specific changes to the code. Just because Homebrew is installed on a system does not mean that any compilation of grok should end up in the Homebrew prefix. It would be reasonable to expect any Homebrew formula for grok to set the prefix correctly.