Fix libsnark dependency build.

This changes libsnark to build in-place, instead of copying first to
a build directory. Previously, modifications made to the original
sources wouldn't get rebuilt without a 'make clean' because users
would be pointing to the copies.

This closes #2689.

src/snark/libsnark/common/template_utils.hpp

@@ -0,0 +1,26 @@
+/** @file
+ *****************************************************************************
+
+ Declaration of functions for supporting the use of templates.
+
+ *****************************************************************************
+ * @author     This file is part of libsnark, developed by SCIPR Lab
+ *             and contributors (see AUTHORS).
+ * @copyright  MIT license (see LICENSE file)
+ *****************************************************************************/
+
+#ifndef TEMPLATE_UTILS_HPP_
+#define TEMPLATE_UTILS_HPP_
+
+namespace libsnark {
+
+/* A commonly used SFINAE helper type */
+template<typename T>
+struct void_type
+{
+    typedef void type;
+};
+
+} // libsnark
+
+#endif // TEMPLATE_UTILS_HPP_