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/fields/field_utils.hpp

@@ -0,0 +1,51 @@
+/** @file
+ *****************************************************************************
+ * @author     This file is part of libsnark, developed by SCIPR Lab
+ *             and contributors (see AUTHORS).
+ * @copyright  MIT license (see LICENSE file)
+ *****************************************************************************/
+
+#ifndef FIELD_UTILS_HPP_
+#define FIELD_UTILS_HPP_
+#include <cstdint>
+
+#include "common/utils.hpp"
+#include "algebra/fields/bigint.hpp"
+
+namespace libsnark {
+
+// returns root of unity of order n (for n a power of 2), if one exists
+template<typename FieldT>
+FieldT get_root_of_unity(const size_t n);
+
+template<typename FieldT>
+std::vector<FieldT> pack_int_vector_into_field_element_vector(const std::vector<size_t> &v, const size_t w);
+
+template<typename FieldT>
+std::vector<FieldT> pack_bit_vector_into_field_element_vector(const bit_vector &v, const size_t chunk_bits);
+
+template<typename FieldT>
+std::vector<FieldT> pack_bit_vector_into_field_element_vector(const bit_vector &v);
+
+template<typename FieldT>
+std::vector<FieldT> convert_bit_vector_to_field_element_vector(const bit_vector &v);
+
+template<typename FieldT>
+bit_vector convert_field_element_vector_to_bit_vector(const std::vector<FieldT> &v);
+
+template<typename FieldT>
+bit_vector convert_field_element_to_bit_vector(const FieldT &el);
+
+template<typename FieldT>
+bit_vector convert_field_element_to_bit_vector(const FieldT &el, const size_t bitcount);
+
+template<typename FieldT>
+FieldT convert_bit_vector_to_field_element(const bit_vector &v);
+
+template<typename FieldT>
+void batch_invert(std::vector<FieldT> &vec);
+
+} // libsnark
+#include "algebra/fields/field_utils.tcc"
+
+#endif // FIELD_UTILS_HPP_