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/gadgetlib1/gadget.tcc

@@ -0,0 +1,23 @@
+/** @file
+ *****************************************************************************
+ * @author     This file is part of libsnark, developed by SCIPR Lab
+ *             and contributors (see AUTHORS).
+ * @copyright  MIT license (see LICENSE file)
+ *****************************************************************************/
+
+#ifndef GADGET_TCC_
+#define GADGET_TCC_
+
+namespace libsnark {
+
+template<typename FieldT>
+gadget<FieldT>::gadget(protoboard<FieldT> &pb, const std::string &annotation_prefix) :
+    pb(pb), annotation_prefix(annotation_prefix)
+{
+#ifdef DEBUG
+    assert(annotation_prefix != "");
+#endif
+}
+
+} // libsnark
+#endif // GADGET_TCC_