Played around with this. its nice But out of the box It puts the tex off the button. to get it to draw the text correctly I had to modify the draw texts lines
al_draw_text(mainmenu.buttonFont, buttonTextColor, gameWidth / 2 , mainmenu.y + buttonFontSize, ALLEGRO_ALIGN_CENTER, "START");
and remove the portion of code + buttonFontSize for each line that drew a button.
Also straight of of the box the background image is too small (your res art folder) But that was an easy fix.