Makefile: link with libiconv if NEEDS_LIBICONV is defined

This seems to be needed to compile on cygwin.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
diff --git a/Makefile b/Makefile
index 6abd82a..dbc34a2 100644
--- a/Makefile
+++ b/Makefile
@@ -19,6 +19,11 @@
 	ui-snapshot.o ui-blob.o ui-tag.o ui-refs.o
 
 
+ifdef NEEDS_LIBICONV
+	EXTLIBS += -liconv
+endif
+
+
 .PHONY: all git install clean distclean force-version get-git
 
 all: cgit git