Modify Autotools scripts

Modify Autotools scripts to output shared library.

src/main.c

@@ -5,3 +5,16 @@
  */
 
 #include "main.h"
+#include <stdio.h>
+
+/* This file is included in the repository since using a Git submodule would
+ * requried cloning the entire Godot source. */
+#include "gdextension_interface.h" /* GDExtension header for Godot. */
+
+/*
+int main()
+{
+    printf("Starting./n");
+    return(0);
+}
+*/