os_version man page
os_version, os_revision — Version of the OS currently running. Allegro game programming library.
extern int os_version;
extern int os_revision;
The major and minor version of the Operating System currently running. Set by allegro_init(). If Allegro for some reason was not able to retrieve the version of the Operating System, os_version and os_revision will be set to -1. For example: Under Win98 SE (v4.10.2222) os_version will be set to 4 and os_revision to 10.
os_revision(3) is an alias of os_version(3).