m4/sage_spkg_collect.m4: Mention sagelib in the comment

Author: Matthias Koeppe

m4/sage_spkg_collect.m4

@@ -261,8 +261,10 @@ for DIR in $SAGE_ROOT/build/pkgs/*; do
         # sage repository (and thus the release tarball). As a result,
         # we don't need to download the sources, which is what
         # "in_sdist" really means. At the time of this writing, the
-        # only standard script package is sage_conf, and its source is
-        # included under build/pkgs/sage_conf.
+        # only standard script packages are sage_conf and sagelib.
+        # The source of sage_conf is included under build/pkgs/sage_conf/src,
+        # and the source of sagelib is provided by symlinks in
+        # build/pkgs/sagelib/src.
         in_sdist=no
     else
         SPKG_SOURCE=normal