fix spandsp build from cross compile change

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@15869 d0543943-73ff-0310-b7d9-9358b9ac24b2

libs/spandsp/src/Makefile.am

@@ -291,7 +291,7 @@ make_at_dictionary$(EXEEXT): $(top_srcdir)/src/make_at_dictionary.c
 	$(CC_FOR_BUILD) -o make_at_dictionary$(EXEEXT) $(top_srcdir)/src/make_at_dictionary.c  -DHAVE_CONFIG_H 
 
 make_modem_filter$(EXEEXT): $(top_srcdir)/src/make_modem_filter.c $(top_srcdir)/src/filter_tools.c 
-	$(CC_FOR_BUILD) -o make_modem_filter$(EXEEXT) $(top_srcdir)/src/make_modem_filter.c $(top_srcdir)/src/filter_tools.c -DHAVE_CONFIG_H
+	$(CC_FOR_BUILD) -o make_modem_filter$(EXEEXT) $(top_srcdir)/src/make_modem_filter.c $(top_srcdir)/src/filter_tools.c -DHAVE_CONFIG_H -lm
 
 # We need to run make_at_dictionary, so it generates the
 # at_interpreter_dictionary.h file