can we use a gui like qt with allegro? for example, if i want to include buttons and other stuff in my game?
No.
Use the included allegro-gui, guichan, CEGUI or CGUI.
If you have a lot of experience with Qt, you probably can use it with Allegro probably.
One way would be to implement a QPaintEngine and QPaintDevice to render using allegro itself. Another would be to implement an Allegro widget in Qt.
Depends on how much GUI elements you want in your game. If it's simple as a button just do that in code. If it's list boxes, menus, movable windows, etc. then you will most likely want to look into MASkinGUI or guichan. I actually wrote a GUI for allegro long ago and it has since died as a project but I was really close to full GUI functionality and it was damn fast and skinnable.... memories 
Use the included allegro-gui, guichan, CEGUI or CGUI.
The CGUI toolkit uses an object oriented ECMAScript library to create SVG based Custom GUI elements inside a web browser. These widgets can be embedded in an html document and viewed within the html, or the user can open the SVG document in their web browser and view the SVG directly.
That really doesn't sound suitable for writing Allegro applications.
I actually wrote a GUI for allegro long ago and it has since died as a project but I was really close to full GUI functionality and it was damn fast and skinnable.... memories
NAS GUI right? It's a shame you abandoned it, it was a good system.
No.
Use the included allegro-gui, guichan, CEGUI or CGUI.
Is there a CEGUI renderer for Allegro? Or are you just using its OGL renderer with AllegroGL?
Is there a CEGUI renderer for Allegro?
No.