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/scalar_multiplication/wnaf.hpp

@@ -0,0 +1,39 @@
+/** @file
+ *****************************************************************************
+
+ Declaration of interfaces for wNAF ("width-w Non-Adjacent Form") exponentiation routines.
+
+ *****************************************************************************
+ * @author     This file is part of libsnark, developed by SCIPR Lab
+ *             and contributors (see AUTHORS).
+ * @copyright  MIT license (see LICENSE file)
+ *****************************************************************************/
+
+#ifndef WNAF_HPP_
+#define WNAF_HPP_
+
+namespace libsnark {
+
+/**
+ * Find the wNAF representation of the given scalar relative to the given window size.
+ */
+template<mp_size_t n>
+std::vector<long> find_wnaf(const size_t window_size, const bigint<n> &scalar);
+
+/**
+ * In additive notation, use wNAF exponentiation (with the given window size) to compute scalar * base.
+ */
+template<typename T, mp_size_t n>
+T fixed_window_wnaf_exp(const size_t window_size, const T &base, const bigint<n> &scalar);
+
+/**
+ * In additive notation, use wNAF exponentiation (with the window size determined by T) to compute scalar * base.
+ */
+template<typename T, mp_size_t n>
+T opt_window_wnaf_exp(const T &base, const bigint<n> &scalar, const size_t scalar_bits);
+
+} // libsnark
+
+#include "algebra/scalar_multiplication/wnaf.tcc"
+
+#endif // WNAF_HPP_