Use LN variable instead of ln -s in Makefile

Makefile

@@ -108,7 +108,7 @@ plugin-soft: prepare-plugin
 	$(call soft-install, "$(VIMPATH)/pack", "$(PWD)/pack")
 	$(call update-ycm)
 	@if [[ ! -f /usr/bin/libtinfo.so.5 ]]; then \
-		sudo ln -s /usr/lib/libncurses.so /usr/lib/libtinfo.so.5; \
+		sudo $(LN) /usr/lib/libncurses.so /usr/lib/libtinfo.so.5; \
 	fi
 
 plugin-hard: prepare-plugin
@@ -116,7 +116,7 @@ plugin-hard: prepare-plugin
 	$(call hard-install, "$(VIMPATH)/pack", "$(PWD)/pack/"*)
 	$(call update-ycm)
 	@if [[ ! -f /usr/bin/libtinfo.so.5 ]]; then \
-		sudo ln -s /usr/lib/libncurses.so /usr/lib/libtinfo.so.5; \
+		sudo $(LN) /usr/lib/libncurses.so /usr/lib/libtinfo.so.5; \
 	fi
 
 plugin: prepare-plugin