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);