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/curves/curve_utils.tcc

@@ -0,0 +1,37 @@
+/** @file
+ *****************************************************************************
+ * @author     This file is part of libsnark, developed by SCIPR Lab
+ *             and contributors (see AUTHORS).
+ * @copyright  MIT license (see LICENSE file)
+ *****************************************************************************/
+
+#ifndef CURVE_UTILS_TCC_
+#define CURVE_UTILS_TCC_
+
+namespace libsnark {
+
+template<typename GroupT, mp_size_t m>
+GroupT scalar_mul(const GroupT &base, const bigint<m> &scalar)
+{
+    GroupT result = GroupT::zero();
+
+    bool found_one = false;
+    for (long i = scalar.max_bits() - 1; i >= 0; --i)
+    {
+        if (found_one)
+        {
+            result = result.dbl();
+        }
+
+        if (scalar.test_bit(i))
+        {
+            found_one = true;
+            result = result + base;
+        }
+    }
+
+    return result;
+}
+
+} // libsnark
+#endif // CURVE_UTILS_TCC_