Properly handle when header file is not found

When header file is not found, generate a dummy Makefile and target
result so extension build will be successful but essentially a no-op

ext/vagrant_ssl/extconf.rb

@@ -8,4 +8,11 @@ if have_header("openssl/opensslv.h")
   end
   append_ldflags(["-lssl"])
   create_makefile("vagrant_ssl")
+else
+  # If the header file isn't found, just create a dummy
+  # Makefile and stub the library to make it a noop
+  File.open("Makefile", "wb") do |f|
+    f.write(dummy_makefile(__dir__).join("\n"))
+  end
+  FileUtils.touch("vagrant_ssl.so")
 end