I didn't build it and run it since I don't have allegro, but I glanced over it real quick.
Just a couple styling questions:
Why are D_WIDTH and D_HEIGHT defines? It would be better (style wise) to use constants like you did for your FPS variable.
For variables like scor and diflevel, you may want to consider using vectors so you can add more amounts of score? and difficulty with ease.
For the next step in the project I would sort things into classes and more functions instead of having everything in main. Looks good for a beginner, although I didn't play it and just glanced through it.
I never actually plan things out, Improv all the way, anyways so I defined them first just for fun and when I reached to timer, I initialized FPS, you see, when I finished the project I was like "You know what it needs?, a difficulty" so I just create integers at random places and hope it doesn't mess up.
You see, I just wanted this game to "Work" , the collision is also messed up sometimes, but works.
And, Now i'll take it more seriously and will plan and break the project into modules.
Thanks for the tips! Have a good day.