build against the system ltdl rather than the local copy

--- ski-1.3.2/src/Makefile.am
+++ ski-1.3.2/src/Makefile.am
@@ -181,8 +181,7 @@
 	eparse.h \
 	libdas.h \
 	netdev.h \
-	osload.h \
-	ltdl.h
+	osload.h 
 
 EXTRA_linux_FILES = \
 	linux/dwarf-linux.c \
@@ -279,7 +278,6 @@
 	eparse.y \
 	escan.l \
 	load.c \
-	ltdl.c \
 	platform.c \
 	libdas.c \
 	libsrs.c