[glibutil] Initialize default log settings from the environment

The following environment variables are checked on Linux:

  GUTIL_LOG_DEFAULT_LEVEL - configures default log level
  GUTIL_LOG_TIMESTAMP     - configures gutil_log_timestamp option
  GUTIL_LOG_TID           - configures gutil_log_tid option

This setup is done once at program's startup and then the program
can override these settings the way it wants, based on its command
line options, configuration files or whatever.

Allows to add thread id prefix to the log messages. That can be useful
for debugging race conditions and other multi-threaded issues.

src/gutil_log.c

@@ -31,6 +31,9 @@
  */
 
 #include "gutil_log.h"
+#include "gutil_misc.h"
+
+#include <stdlib.h>
 
 #ifdef unix
 #  include <unistd.h>
@@ -639,6 +642,33 @@ gutil_log_get_type()
                                                   GLOG_TYPE_CUSTOM;
 }
 
+/* Initialize defaults from the environment */
+#ifndef _WIN32
+__attribute__((constructor))
+static
+void
+gutil_log_init()
+{
+    int val = 0;
+
+    if (gutil_parse_int(getenv("GUTIL_LOG_DEFAULT_LEVEL"), 0, &val) &&
+        val >= GLOG_LEVEL_INHERIT && val <= GLOG_LEVEL_VERBOSE) {
+        gutil_log_default.level = val;
+        GDEBUG("Default log level %d", val);
+    }
+
+    if (gutil_parse_int(getenv("GUTIL_LOG_TIMESTAMP"), 0, &val) && val > 0) {
+        gutil_log_timestamp = (val > 0);
+        GDEBUG("Timestamps %s", (val > 0) ? "enabled" : "disabled");
+    }
+
+    if (gutil_parse_int(getenv("GUTIL_LOG_TID"), 0, &val) && val > 0) {
+        gutil_log_tid = (val > 0);
+        GDEBUG("Thread id prefix %s", (val > 0) ? "enabled" : "disabled");
+    }
+}
+#endif
+
 /*
  * Local Variables:
  * mode: C