Install script checks for php-dom vs php-xml

[plietar]

Hi,
I've noticed an issue in the extension checks done by the install script :
If the dom extension is missing, the install script asks the user to install the xml extension.
If the dom extension is present but the xml is not, the install script happily proceeds, but retrieving feeds will fail as SimplePie requires it.

Both extensions are required by FreshRSS I believe, and should have their own check.

[Alkarex]

Thanks for the bug report. I thought the XML extension was always available, but indeed, it is possible to explicitly opt-out http://php.net/manual/xml.installation.php

If you have time, it would be great if you could make a pull request (otherwise, I will try to add a check before the next beta). See:

• https://github.com/FreshRSS/FreshRSS/blob/dev/app/install.php#L310
• http://simplepie.org/wiki/setup/requirements

[Alkarex]

Check added in #1097