Use INSTALL_NAME_TOOL and OTOOL environment variables in plugin makefile on Mac.

compile.mk

@@ -6,6 +6,8 @@ include $(RACK_DIR)/arch.mk
 
 OBJCOPY ?= objcopy
 STRIP ?= strip
+INSTALL_NAME_TOOL ?= install_name_tool
+OTOOL ?= otool
 
 # Generate dependency files alongside the object files
 FLAGS += -MMD -MP

plugin.mk

@@ -67,9 +67,9 @@ dist: all
 	cp $(TARGET) dist/$(SLUG)/
 ifdef ARCH_MAC
 	$(STRIP) -S dist/$(SLUG)/$(TARGET)
-	install_name_tool -change libRack.dylib @rpath/libRack.dylib dist/$(SLUG)/$(TARGET)
-	install_name_tool -add_rpath . dist/$(SLUG)/$(TARGET)
-	otool -L dist/$(SLUG)/$(TARGET)
+	$(INSTALL_NAME_TOOL) -change libRack.dylib @rpath/libRack.dylib dist/$(SLUG)/$(TARGET)
+	$(INSTALL_NAME_TOOL) -add_rpath . dist/$(SLUG)/$(TARGET)
+	$(OTOOL) -L dist/$(SLUG)/$(TARGET)
 else
 	$(STRIP) -s dist/$(SLUG)/$(TARGET)
 endif