Do not run libtoolize by autogen.sh if libtoolize is missing

(fix of commit c9ebc16)

* autogen.sh: Do not run libtoolize if it is not available (i.e. which
command cannot find it); update comment.

autogen.sh

@@ -6,7 +6,12 @@ set -e
 
 # Install libtool.m4 and ltmain.sh in the build tree.  This command is needed
 # if autoreconf and libtoolize are available from the different directories.
-libtoolize -i -c
+# Note: libtoolize might be missing on some platforms.
+if which libtoolize > /dev/null 2>&1; then
+  libtoolize -i -c
+else
+  echo "libtoolize is not found, ignoring!"
+fi
 
 autoreconf -i