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/algebra/exponentiation/exponentiation.hpp

@@ -0,0 +1,31 @@
+/** @file
+ *****************************************************************************
+
+ Declaration of interfaces for (square-and-multiply) exponentiation.
+
+ *****************************************************************************
+ * @author     This file is part of libsnark, developed by SCIPR Lab
+ *             and contributors (see AUTHORS).
+ * @copyright  MIT license (see LICENSE file)
+ *****************************************************************************/
+
+#ifndef EXPONENTIATION_HPP_
+#define EXPONENTIATION_HPP_
+
+#include <cstdint>
+
+#include "algebra/fields/bigint.hpp"
+
+namespace libsnark {
+
+template<typename FieldT, mp_size_t m>
+FieldT power(const FieldT &base, const bigint<m> &exponent);
+
+template<typename FieldT>
+FieldT power(const FieldT &base, const unsigned long exponent);
+
+} // libsnark
+
+#include "algebra/exponentiation/exponentiation.tcc"
+
+#endif // EXPONENTIATION_HPP_