log: print build target triplet alongside with the version

This is useful for example for the analysis of performance complaints
from users, when they claim that one version of Tarantool is slower
than another, in fact comparing debug and release builds.

NO_DOC=minor change
NO_TEST=minor change

changelogs/unreleased/log-build-target-triplet.md

+## feature/core
+
+* Now the log produced during `box.cfg{}` contains the build target triplet
+  (e.g. `Linux-x86_64-RelWithDebInfo`).

src/main.cc

@@ -501,7 +501,8 @@ load_cfg(void)
 	 * after (optional) daemonising to avoid confusing messages with
 	 * different pids
 	 */
-	say_info("%s %s", tarantool_package(), tarantool_version());
+	say_info("%s %s %s", tarantool_package(), tarantool_version(),
+		 BUILD_INFO);
 	say_info("log level %i", cfg_geti("log_level"));
 
 	if (pid_file_handle != NULL) {