Provide --no-tcl option

[adamnbowen]

I just tried installing postgresql via Boxen on a fresh 10.9 computer, and it seems that the tclConfig.sh file doesn't exist in a place where the build process can find it. Homebrew provides a --no-tcl option for building postgresql—any chance we could add that switch into this project, through something like include postgresql::no_tcl?

I discovered the solution by looking at this issue on the homebrew repo: Homebrew/legacy-homebrew#21099