al_mouse_button_down - Allegro 5 API

       Return true if the mouse button specified was held down in the state specified.

       Unlike   most  things,  the  first  mouse  button  is  numbered  1.   As  a  convenience,  the  constants
       ALLEGRO_MOUSE_BUTTON_LEFT,   ALLEGRO_MOUSE_BUTTON_RIGHT,   ALLEGRO_MOUSE_BUTTON_MIDDLE   are    provided.
       However,  depending  on  the  hardware  there  may  be  more  or  fewer  mouse  buttons.   You  can check
       al_get_mouse_num_buttons(3alleg5) if you want to be sure.

       al_mouse_button_down - Allegro 5 API

ALLEGRO_MOUSE_STATE(3alleg5), al_get_mouse_state(3alleg5), al_get_mouse_state_axis(3alleg5)

Allegro reference manual                                                           al_mouse_button_down(3alleg5)

              #include <allegro5/allegro.h>

              bool al_mouse_button_down(const ALLEGRO_MOUSE_STATE *state, int button)