PROTON-445: PHP install now honors the install prefix.

If the install prefix is not "/usr" then the INI directory is placed
below it. This is to accomodate a system install as opposed to a
developer install.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1547597 13f79535-47bb-0310-9956-ffa450edef68