These functions are declared in the main Allegro header file:
#define ALLEGRO_PI 3.14159265358979323846
C99 compilers have no predefined value like M_PI for the constant π, but you can use this one instead.
int al_run_main(int argc, char **argv, int (*user_main)(int, char **))
This function is useful in cases where you don't have a main() function but want to run Allegro (mostly useful in a wrapper library). Under Windows and Linux this is no problem because you simply can call al_install_system. But some other system (like OSX) don't allow calling al_install_system in the main thread. al_run_main will know what to do in that case.
The passed argc and argv will simply be passed on to user_main and the return value of user_main will be returned.