Allow specifying natives directory with the fabric.loom.natives.dir gradle property.

Note this is designed to allow easy usage of custom natives, thus will not populate the directory with the default natives.

src/main/java/net/fabricmc/loom/LoomGradleExtension.java

@@ -236,6 +236,12 @@ public class LoomGradleExtension {
 	}
 
 	public File getNativesDirectory() {
+		Object customNativesDir = project.getProperties().get("fabric.loom.natives.dir");
+
+		if (customNativesDir != null) {
+			return new File((String) customNativesDir);
+		}
+
 		File natives = new File(getUserCache(), "natives/" + getMinecraftProvider().getMinecraftVersion());
 
 		if (!natives.exists()) {
@@ -245,6 +251,10 @@ public class LoomGradleExtension {
 		return natives;
 	}
 
+	public boolean hasCustomNatives() {
+		return project.getProperties().get("fabric.loom.natives.dir") != null;
+	}
+
 	public File getDevLauncherConfig() {
 		return new File(getProjectPersistentCache(), "launch.cfg");
 	}

src/main/java/net/fabricmc/loom/providers/MinecraftNativesProvider.java

@@ -45,6 +45,14 @@ public class MinecraftNativesProvider {
 		File nativesDir = extension.getNativesDirectory();
 		File jarStore = extension.getNativesJarStore();
 
+		if (extension.hasCustomNatives()) {
+			if (!nativesDir.exists()) {
+				throw new RuntimeException("Could no find custom natives directory at " + nativesDir.getAbsolutePath());
+			}
+
+			return;
+		}
+
 		for (MinecraftVersionInfo.Library library : versionInfo.libraries) {
 			File libJarFile = library.getFile(jarStore);