ACF5 Pro licence activation. Could be adapted for any other plugin that requires a licence key, but doesn't yet support defining it in wp-config. Fires on theme activation.

licence activation.php

// Place this in wp-config
define( 'ACF_5_KEY', 'yourkeyhere' );

// Set ACF 5 license key on theme activation. Stick in your functions.php or equivalent.
function auto_set_license_keys() {
  
  if ( ! get_option( 'acf_pro_license' ) && defined( 'ACF_5_KEY' ) ) {
    
    $save = array(
		'key'	=> ACF_5_KEY,
		'url'	=> home_url()
	);
	
	$save = maybe_serialize($save);
	$save = base64_encode($save);
      
    update_option( 'acf_pro_license', $save );
  }
}
add_action( 'after_switch_theme', 'auto_set_license_keys' );