Refined flatbuffers dependency implementation (see #132)

Included flatc.exe within repository (v1.11.0 binary)
Added check within :core:createFlatBuffers task for Windows OS family
Added flatbuffersVersion variable within gradle.properties to 1.11.0
Changed many fields to variables and implemented flatbuffers closure

core/build.gradle

@@ -3,6 +3,7 @@ apply plugin: 'artemis'
 apply plugin: 'io.netifi.flatbuffers'
 
 import io.netifi.flatbuffers.plugin.tasks.FlatBuffers
+import org.gradle.internal.os.OperatingSystem
 
 [compileJava, compileTestJava]*.options*.encoding = 'UTF-8'
 
@@ -13,6 +14,8 @@ project.ext.generatedSourceDirs = file('gen/main/java/')
 sourceSets.main.java.srcDirs += generatedSourceDirs
 idea.module.generatedSourceDirs += generatedSourceDirs
 
+project.ext.flatbuffersSourceDirs = file('src/main/java/com/riiablo/net/')
+
 dependencies {
   api "com.badlogicgames.gdx:gdx:$gdxVersion"
   api "com.badlogicgames.gdx:gdx-backend-lwjgl:$gdxVersion"
@@ -29,7 +32,7 @@ dependencies {
   api "org.apache.commons:commons-math3:3.6.1"
   api "org.apache.commons:commons-text:1.8"
   api "com.jcraft:jzlib:1.1.3"
-  api "com.google.flatbuffers:flatbuffers-java:1.11.0"
+  api "com.google.flatbuffers:flatbuffers-java:$flatbuffersVersion"
 }
 
 dependencies {
@@ -81,8 +84,19 @@ dependencies {
 //
 // classes.finalizedBy weave
 
-task createFlatBuffers(type: FlatBuffers) {
-  inputDir = file('src/main/java/com/riiablo/net/')
-  outputDir = file('gen/main/java/')
+flatbuffers {
+  flatcPath = rootProject.file('flatbuffers/flatc.exe')
   language = 'java'
+  flatBuffersVersion = flatbuffersVersion
+}
+
+task createFlatBuffers(type: FlatBuffers) {
+  doFirst {
+    if (OperatingSystem.current() != OperatingSystem.WINDOWS) {
+      throw new GradleException("'flatc' can only run on Windows")
+    }
+  }
+
+  inputDir = flatbuffersSourceDirs
+  outputDir = generatedSourceDirs
 }

gradle.properties

@@ -9,5 +9,6 @@ artemisOdbVersion=2.3.0
 artemisContribVersion=2.4.0
 gdxAiVersion=1.8.2
 nettyVersion=4.1.50.Final
+flatbuffersVersion=1.11.0
 cliVersion=1.4
 box2DLightsVersion=1.5