build: fix undefined macro: AM_PROG_LIBTOOL with recent libtool

Without this patch, I get:

 configure.ac:206: warning: macro `AM_PROG_LIBTOOL' not found in library
 configure.ac:213: error: possibly undefined macro: AM_PROG_LIBTOOL
       If this token and others are legitimate, please use m4_pattern_allow.
       See the Autoconf documentation.
 autoreconf2.50: /usr/bin/autoconf failed with exit status: 1

(I am using libltdl-dev 2.2.6a-4 from Debian)

1 files changed, 1 insertions, 0 deletions

diff --git a/autogen.sh b/autogen.sh
index 56183be..a7c8129 100755
--- a/autogen.sh
+++ b/autogen.sh
@@ -131,6 +131,7 @@ if test -z "$*"; then
 fi
 
 
+libtoolize --force || echo "libtoolize failed"
 gtkdocize || echo "gtkdocize failed"
 
 export AUTOMAKE ACLOCAL