[AD] configurable logging

[Elias Pschernig <elias.pschernig@xxxxxxxxxx>]

I find our log files a bit hard to read lately with all the GL 
extensions and X11 keyboard codes in it. At the same time, I think we 
log way too few things yet to properly debug some problems. Therefore I 
think we should have somewhat more advanced logging. I found the way it 
works in Wine is very simple yet can provide really detailed information 
when needed for hard to track bug reports, so I modeled the following 
proposal after how they do it.

The first thing would be a way to select what you are interested in, for 
us this should be via allegro.cfg:

[trace]
channels=all
level=info
timestamps=0
lines=0

The channels key tells which channels to log, default would be all, can 
set to none to disable logging, or else a comma separated list of 
system, display, keyboard, x11, font,... each source file or addon can 
define their own channels if they want.

The level key specifies how verbosely to log:
error: Only errors are logged.
warn: The above, plus also warnings.
info: Above, plus important info. About what we have right now.
debug: Detailed tracing. E.g. things like loading a bitmap
       successfully - this would be off by default, but we might
       want to ask for it in hard to reproduce bug reports.

With "timestamps" could select whether to prepend log messages with a 
timestamp. It will make it easy to spot badly written parts in our 
startup/shutdown code - off by default.

And finally whether to prepend log messages with file:linenumber, just 
because I like to find things faster - but off by default.

The use inside the source code would not change much from how it is 
currently, except the somewhat cumbersome PREFIX_* definitions would not 
exist any longer. At the beginning of each source file, we would instead 
have something like:

ALLEGRO_DEBUG_CHANNEL("display")

And TRACE would be split into four functions:

ALLEGRO_ERROR("No display can be created.\n");

ALLEGRO_WARN("Selected format %s not usable by Allegro.\n", format);

ALLEGRO_INFO("Choosing visual %d.\n", i);

ALLEGRO_TRACE("OpenGL extension %s loaded successfully.\n", name);

--
Elias Pschernig <elias@xxxxxxxxxx>