backport newer libtool logic

https://bugs.gentoo.org/539032

--- a/configure.ac
+++ b/configure.ac
@@ -464,28 +464,15 @@
 dnl -----------------------------------
 dnl libtool ltdl on-demand plugin loading
 
-m4_ifdef([LT_INIT],
-    [ #code that is for Libtool 2.x
-AM_PROG_LIBTOOL
-    ],
-    [ #code that is for 1.5.x
-    ])
+LT_INIT([dlopen])
+LT_CONFIG_LTDL_DIR([libltdl])
+LTDL_INIT
+
 AC_ARG_ENABLE(ltdl,
   [AS_HELP_STRING([--enable-ltdl],[support on-demand plugin loading])])
 if test "x$enable_ltdl" != "xno"; then
   AC_DEFINE(ENABLE_LTDL,1,[Define if you want on-demand plugin loading])
-  AC_LIBTOOL_DLOPEN
-m4_ifdef([LT_INIT],
-    [ #code that is for Libtool 2.x
-  LT_CONFIG_LTDL_DIR([libltdl])
-  LTDL_INIT
-    ],
-    [ #code that is for 1.5.x
-  AC_CONFIG_SUBDIRS([libltdl])
-    ])
   use_ltdl="Yes"
-  
-  AC_LIBLTDL_CONVENIENCE
   if test "x$DARWIN9" = "xyes"; then
      LIBLTDL_LDFLAGS="-Wl,-unexported_symbol,_lt_*"
   fi
@@ -494,12 +483,6 @@
 AC_SUBST(INCLTDL)
 AC_SUBST(LIBLTDL)
 AC_SUBST(LIBLTDL_LDFLAGS)
-m4_ifdef([LT_INIT],
-    [ #code that is for Libtool 2.x
-    ],
-    [ #code that is for 1.5.x
-AM_PROG_LIBTOOL
-    ])
 
 # Checks for libraries.
 #AC_CHECK_LIB([ICE], [main])