mysql: strip invalid flags from mysql_config.

regex in previous commit did not strip all invalid flags and stripped out an additional flag "-Wall." this commit resolves this.

previous commit: https://github.com/vito/homebrew/commit/9dd1c38a73597535356cb2053f52ca7ef253ff79
discussion: https://github.com/mxcl/homebrew/issues/20788#issuecomment-21242725