[make] Set rpath on mac when building haxelib

It looks like /usr/local/lib is no longer used as a default path in some cases.

Makefile

@@ -104,13 +104,19 @@ copy_haxetoolkit: /cygdrive/c/HaxeToolkit/haxe/haxe.exe
 	cp $< $@
 endif
 
-# haxelib should depends on haxe, but we don't want to do that...
+ifeq ($(SYSTEM_NAME),Mac)
+# This assumes that haxelib and neko will both be installed into INSTALL_DIR,
+# which is the case when installing using the mac installer package
+HAXELIB_LFLAGS= -Wl,-rpath,$(INSTALL_DIR)/lib
+endif
+
 haxelib_unix:
 	cd $(CURDIR)/extra/haxelib_src && \
 	HAXE_STD_PATH=$(CURDIR)/std $(CURDIR)/$(HAXE_OUTPUT) client.hxml && \
 	nekotools boot -c run.n
-	${CC} $(CURDIR)/extra/haxelib_src/run.c -o $(HAXELIB_OUTPUT) -lneko
+	$(CC) $(CURDIR)/extra/haxelib_src/run.c -o $(HAXELIB_OUTPUT) -lneko $(HAXELIB_LFLAGS)
 
+# haxelib should depends on haxe, but we don't want to do that...
 ifeq ($(SYSTEM_NAME),Windows)
 haxelib: haxelib_$(PLATFORM)
 else