Yeah, I'm aware it takes a lot more time, but it's useful for people who don't have the registered version, or whatever. And there's no glitches in it, although there's probably a fix to the one in yours.
EDIT: I was bored, and combined both of our scripts to achive a glitch-free and very fast line thickness script:
//draw_line_thickness(x1,y1,x2,y2,thickness);
var thickness, startX, startY, endX, endY, tX;
startX = argument0;
startY = argument1;
endX = argument2;
endY = argument3;
thickness = argument4; //in pixels
tX = point_distance(startX,startY,endX,endY)
draw_sprite_ext(sprLine,0,startX+lengthdir_x(thickness/2,point_direction(startX,startY,endX,endY)+90),startY+lengthdir_y(thickness/2,point_direction(startX,startY,endX,endY)+90),tX,thickness,point_direction(startX,startY,endX,endY),c_white,1);
draw_circle(startX,startY,thickness/2,0)
draw_circle(endX,endY,thickness/2,0)