SKaero I can get you a little further.
If you're using IIS you need to use the non thread safe version of PHP. I was doing testing on Windows 10 Pro x64, IIS, with PHP 7.1 x64 (Non Thread Safe), and MSSQL 2016 Express(ODBC 13).
These PDO drivers work.
https://github.com/Microsoft/msphpsql/releasesSpecifically
https://github.com/Microsoft/msphpsql/releases/tag/4.1.5-Windows7.1 Driver
Place the drivers in your extension directory specified in the php.ini and add the following entries to your php.ini
extension=php_sqlsrv_71_nts.dll
extension=php_pdo_sqlsrv_71_nts.dll
Then the SQL Server (SQLSRV) will become available and the installer script will flag it as OK, however when testing the config (step 3) I kept getting issue with the roundcube installer script saying that the database was not initialized. throwing an ODBC 13 error I would provide it but I dont actively have it accessible at the moment that I'm writing this. I tried running the database creation (.sql) script directly on the server hoping that would initialize the database appropriately.
As far as SQL is concerned I created a separate user for running roundcube exclusively. Setting both owner and dbcreator permissions for the roundcube database initially. Later in a vain effort to get it working I temporarily granted it sysadmin to no avail as a test.
Then I though perhaps the PDO driver was not working or installed correctly. So i tested it with a small PHP script to connect and return records from a database this worked without issue so it seems to be something particular to roundcubes installer script where it is failing.
One thing i will mention is when i added the extensions at the time i personally only add "extension=php_pdo_sqlsrv_71_nts.dll" however I've done some additional reading and found that its advised to add both as I mentioned above if you try this and find success let me know otherwise I will likely be testing this this weekend myself.