Add ALLEGRO_TRACE_LEVEL environment variable.

Fixes #1556

allegro5.cfg

@@ -143,8 +143,8 @@ webp_quality_level = lossless
 # device0=/dev/input/by-id/usb-blahblah-joystick
 
 # Windows: You can choose between the XINPUT or DIRECTINPUT driver for
-# joysticks and force feedback joysticks. Xinput is the more modern 
-# system, but DirectInput has more force feedback capabilities for older 
+# joysticks and force feedback joysticks. Xinput is the more modern
+# system, but DirectInput has more force feedback capabilities for older
 # joysticks.
 driver=XINPUT
 
@@ -189,6 +189,7 @@ channels=all
 # If not none, Allegro will write out the logs to an allegro.log file next to
 # the binary. Use ALLEGRO_TRACE environment variable to control that file
 # location. A special filename of - (dash) means logging to stdout.
+# You can override this value via the ALLEGRO_TRACE_LEVEL environment variable.
 level=
 
 # Set to 0 to disable line numbers in log files.

src/debug.c

@@ -136,7 +136,9 @@ void _al_configure_logging(void)
    trace_info.level = 9999;
 #endif
 
-   v = al_get_config_value(config, "trace", "level");
+   v = getenv("ALLEGRO_TRACE_LEVEL");
+   if (!v)
+      v = al_get_config_value(config, "trace", "level");
    if (v) {
       if (!strcmp(v, "error")) trace_info.level = 3;
       else if (!strcmp(v, "warn")) trace_info.level = 2;