ltsmin: use any installed libltdl

This should solve issue with the Debian package.

* spot/ltsmin/Makefile.am: Use the LTDLINC, LTDLDEPS and LIBLTDL as
documented.
* NEWS: Mention the fix.

NEWS

@@ -1,5 +1,10 @@
 New in spot 2.2.1.dev (Not yet released)
 
+  Build:
+
+  * If the system has an installed libltdl library, use it instead of
+    the one we distribute.
+
   Bug fixes:
 
   * scc_filter() had a left-over print statement that would print

spot/ltsmin/Makefile.am

@@ -1,6 +1,6 @@
 ## -*- coding: utf-8 -*-
-## Copyright (C) 2011, 2013, 2014, 2015 Laboratoire de Recherche et
-## Developpement de l'Epita (LRDE).
+## Copyright (C) 2011, 2013, 2014, 2015, 2016 Laboratoire de Recherche
+## et Developpement de l'Epita (LRDE).
 ##
 ## This file is part of Spot, a model checking library.
 ##
@@ -18,7 +18,7 @@
 ## along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 AM_CPPFLAGS = -I$(top_builddir) -I$(top_srcdir) \
-              $(BUDDY_CPPFLAGS) -I$(top_srcdir)/ltdl
+              $(BUDDY_CPPFLAGS) $(LTDLINCL)
 AM_CXXFLAGS = $(WARNING_CXXFLAGS)
 
 ltsmindir = $(pkgincludedir)/ltsmin
@@ -26,8 +26,11 @@ ltsmindir = $(pkgincludedir)/ltsmin
 ltsmin_HEADERS = ltsmin.hh
 
 lib_LTLIBRARIES = libspotltsmin.la
+libspotltsmin_la_DEPENDENCIES = \
+  $(top_builddir)/spot/libspot.la \
+  $(LTDLDEPS)
 libspotltsmin_la_LIBADD = \
   $(top_builddir)/spot/libspot.la \
-  $(top_builddir)/ltdl/libltdlc.la -lpthread
+  $(LIBLTDL) -lpthread
 libspotltsmin_la_LDFLAGS = -no-undefined $(SYMBOLIC_LDFLAGS)
 libspotltsmin_la_SOURCES = ltsmin.cc