Refine the build id string

We use git describe to get the SHA1 so that we append -dirty for dirty
worktrees and include the HEAD commit subject and date.
diff --git a/src/compositor.c b/src/compositor.c
index aa862cd..26c9a95 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -3220,7 +3220,7 @@
 		   STAMP_SPACE "Bug reports to: %s\n"
 		   STAMP_SPACE "Build: %s\n",
 		   PACKAGE_STRING, PACKAGE_URL, PACKAGE_BUGREPORT,
-		   WESTON_SHA1);
+		   BUILD_ID);
 	log_uname();
 
 	display = wl_display_create();