I also managed to build the AGS allegro library with a newer compiler in the past, and don't remember any problems.
You still have the workaround of installing VC9 before VS 2012 and compile the project with the old compiler (simply choose it in the project settings). I checked this with the VC10 compiler and it works.
You still have the workaround of installing VC9 before VS 2012 and compile the project with the old compiler (simply choose it in the project settings). I checked this with the VC10 compiler and it works.