Autoconf, Automake and Libtool

We need these now for Xcode-4.3/CLT4X installations.

Also prevent m4 error in installer. And prevent brew doctor complaining if we're Xcode 4.3 or above.

Closes Homebrew/homebrew#10349. Fixes Homebrew/homebrew#10423. Refs Homebrew/homebrew#9179.