Fix Makefile for Windows
The default value of GRADLE is "./gradlew", which is the correct value
on Linux.
On Windows, it should use gradlew.bat (by calling "gradlew") instead.
.PHONY: default release clean build build-app build-server run dist dist-zip sums test
+ifeq ($(OS),Windows_NT)
+ GRADLE ?= gradlew
+else
GRADLE ?= ./gradlew
+endif
APP_BUILD_DIR := app-build
DIST := dist
