[AD] Turning logging on in release builds by default

[SiegeLord <siegelordex@xxxxxxxxxx>]

I spent a little bit of time implementing things to enable logging by 
default in Allegro release builds. By 'logging by default' I mean that 
the logging calls are not compiled out, but nothing is actually logged 
as the release builds default to filtering out all the logging messages. 
This amounts to two branches and a single function call per logging 
statement.

Thus, from the point of view of the users, nothing changes (e.g. there's 
no allegro.log popping out everywhere). However, by placing an 
allegro5.cfg next to the program, you can enable logging without 
recompiling. This seems like a useful feature to have since quite a few 
of our distribution methods don't compile the debug build, and to access 
the debug binaries you need to compile them yourself.

Since adding that allegro5.cfg is a bit of a pain at times, I also made 
it so that you can set the logging level in the system configuration 
before al_install_system. I.e. in your program you can call 
al_set_config_value(al_get_system_config(), "trace", "level", "debug"); 
before al_install_system and enable debugging that way. This required 
changing how the system configuration worked a little bit.

I checked the performance of this, and saw no detectable changes (I 
tested primarily with ex_blit, as that logs about 6-10 times per frame) 
so I don't think that's a concern. You can always compile Allegro 
yourself to disable the logging statements (the final piece of this work 
was simply flipping WANT_RELEASE_LOGGING from 'off' to 'on' in 
CMakeLists.txt) if you're worried about your end users somehow screwing 
something up with this logging.

So what do you all think? Any objections to merging this? The 
implementation is here:

https://github.com/liballeg/allegro5/pull/14/commits

-SL