[AD] get_config_string return value

[Peter Wang <tjaden@xxxxxxxxxx>]

Yesterday I found that `get_config_string' returns the empty string
for a variable if the config file contains "var =".  Is this a
recent change, or has it always been like this?

The worst thing is, it makes it very confusing for people who copy
`allegro.cfg' into their home directories (or wherever), and get
blanks for all the default strings (e.g. mouse_driver, oss_driver,
etc.), so things stop working :-(

So we need to do one of:
- comment out the "var =" lines in allegro.cfg
- make get_config_string return the default string for "var ="
- correct the incorrect uses of `get_config_string'

-- 
tjaden@xxxxxxxxxx - http://www.alphalink.com.au/~tjaden/
ZIBULA (ZI bew luh), n.  The plastic spine which model car parts come
attached to.  -- Rich Hall, "Sniglets"