When imagemagick 6 is installed on macOS using Homebrew it can be a
bit of a faff to get Emacs built with it. The attached patch
automatically adds it to PKG_CONFIG_PATH if Homebrew is detected.
I'm wondering whether these concatenations work if the packages aren't installed. `brew --prefix texinfo 2>/dev/null`/bin results in /bin if texinfo isn't installed, but this might be a bit too implicit and brittle - how certain are we that brew will print the empty string here forever? Maybe it would be better to check the return value of brew and append the path segments only if it has suceeded.