Idempotent build: PDO/pdo

[alfsb]

Some folders on reference/ contains two folders named pdo and PDO, This works file on Linux but is broken on Windows. And may case random broken builds. They are:

• reference/pdo_cubrid
• reference/pdo_pgsql
• reference/pdo_sqlite

Sometimes files are duplicated between two folders, sometimes not. These files need to be erased or moved around until there is only one folder named pdo (other directories where is already occurs use lower case pdo).