Fix 'possibly undefined macro AC_ENABLE_SHARED' autoreconf error

Issue #616 (bdwgc).

Install libtool.m4 and ltmain.sh in the build tree before running
autoreconf.  This is needed if autoreconf and libtoolize are available
from the different directories of $PATH.

* autogen.sh: Run libtoolize (if available); add comment.

autogen.sh

@@ -12,6 +12,15 @@ set -e
 # prefix.  Otherwise, you may need to invoke autoreconf with the appropriate
 # -I options to locate the required *.m4 files.
 
+# Install libtool.m4 and ltmain.sh in the build tree.  This command is needed
+# if autoreconf and libtoolize are available from the different directories.
+# Note: libtoolize might be missing on some platforms.
+if (type libtoolize) > /dev/null 2>&1; then
+  libtoolize -i -c
+else
+  echo "libtoolize is not found, ignoring!"
+fi
+
 autoreconf -i
 
 echo