Here's a tutorial comparing different windows: http://www.youtube.com/watch?v=Ztt-b8m6YIE

Anyway, just make a target and put it in the glass (or parent it to the player), then make the turret target that, and add a trigger in front of the glass to make the turret firebullet

Alexander Bell wrote:
But the turret can't see through it, from what I understand. Well, it can see through it but it acts like it does when it sees you through glass (which is some nodraw and clip brushes with a special texture, I'm pretty sure)

Right, but turrets shoot glass if the glass is a func_brush, which is odd to say the least.
The main event in the map is that the player hits a button causing turrets being released from the facility. The turrets are to target the player in an office and shoot at the player who happens to be in front of a few window that will shatter when the turret shoots the window while targeting the player.
The room

These capsules hold the turrets (an early picture). The office on the other side of the windows is where the player will be shot at, with reasonable cover.
Func_breakable may work. Tomorrow, I'll try func_breakable. Hopefully, it'll work.