Possible workaround for installing nokogiri 1.6.8 on OSX with brewed libxml2 2.9.4

Problem:

The following commands fail on OSX:

    gem install nokogiri
    gem install nokogiri -- --use-system-libraries
    gem install nokogiri -- --use-system-libraries --with-xslt-dir=/usr/local/opt/libxslt --with-xml2-dir=/usr/local/opt/libxml2

Version:

    libxml2: 2.9.4 (installed via homebrew 0.9.9)
    libxslt: 1.1.28
    nokogiri: 1.6.8


If you tried the last command mentioned above, and the following error
message, then this guide might solve your problem:


    ERROR: cannot discover where libxml2 is located on your system. please make sure `pkg-config` is installed.
    
    
The problem I noticed is that the `xmlversion.h` header
file won't be discovered by `extconf.rb` script because the inferred
path of the `include` directory by nokogiri is
`/usr/local/opt/libxml2/include`. But in the libxml2 `2.9.4` version, the path to the header file is
`/usr/local/opt/libxml2/include/libxml2/xmlversion.h`.
Specifying the `include` and `lib` paths explicitly solves the problem:

    gem install nokogiri -- --use-system-libraries --with-xslt-dir=/usr/local/opt/libxslt --with-xml2-include=/usr/local/opt/libxml2/include/libxml2 --with-xml2-lib=/usr/local/opt/libxml2/lib

I'm not sure if this even a bug, either in functionality or documentation.