WIP: [postfix] Build against custom MySQL

Hi, here I would like to discuss two main issues in Postfix compile scripts:

1. Building against custom MySQL path is undocumented, I would like to propose my functioning solution at least in the README
2. AUXLIBS is deprecated in favour of dedicated vars like:
   • . AUXLIBS_MYSQL (http://www.postfix.org/MYSQL_README.html)
   • . AUXLIBS_PGSQL (http://www.postfix.org/PGSQL_README.html)
   • . AUXLIBS_LDAP (http://www.postfix.org/LDAP_README.html)

What is the recommended way to document (1) and fix (2) these issues?