Revert "Make glibtoolize optional, as presumably was the thought"

I was wrong. I didn't remember that I had had to run glibtoolize
myself once back in the days when initially setting up my iOS build
tree on macOS, in order to get the m4 directory with its symlinks to
the libtool-related m4 files in (for me) /opt/gnu/share/aclocal. Even
if libtool is not used in any way when building the iOS app.

This reverts commit 14e998723f.

Change-Id: I474b0f47a2156a30b22be8c39864ebcd09010e21

autogen.sh

@@ -19,10 +19,7 @@ EOF
 if test `uname -s` = Linux; then
     libtoolize || failed "libtool"
 elif test `uname -s` = Darwin; then
-    echo 'Having glibtoolize is mandatory only if you intend to use the alternative "brew" method. See ios/README.' >&2
-    echo 'So unless you plan to do that, it does not matter if glibtoolize is not be found below:' >&2
-    glibtoolize
-    echo 'There. The script now continues. Errors below are fatal.' >&2
+    glibtoolize || failed "Can't find glibtoolize. For instance use the one from https://brew.sh, 'brew install libtool', or build GNU libtool yourself."
 fi
 
 aclocal || failed "aclocal"