Support to install Mroonga with MeCab all at once

Here is the use case:

  % brew install --use-homebrew-mysql --use-homebrew-mecab

Then mecab,mecab-ipadic,groonga package dependencies are resolved all at once.

If you already installed Groonga without MeCab, what happens?

  % brew install mroonga --use-homebrew-mysql --use-homebrew-mecab
  Error: mroonga dependency groonga not installed with:
    --use-homebrew-mecab

Homebrew reports "Please install Groonga with --use-homebrew-mecab option."

NOTE: This fix works after Homebrew #26308 was merged.