man al_have_opengl_extension (1): Allegro 5 API

SYNOPSIS

#include <allegro5/allegro_opengl.h>
bool al_have_opengl_extension(const char *extension)

This function is a helper to determine whether an OpenGL extension is available on the given display or not.

Example:

bool packedpixels = al_have_opengl_extension("GL_EXT_packed_pixels");

If packedpixels is true then you can safely use the constants related to the packed pixels extension.

Returns true if the extension is available; false otherwise.