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/constraint_profiling.hpp

@@ -0,0 +1,42 @@
+/** @file
+ *****************************************************************************
+
+ Declaration of interfaces for profiling constraints.
+
+ *****************************************************************************
+ * @author     This file is part of libsnark, developed by SCIPR Lab
+ *             and contributors (see AUTHORS).
+ * @copyright  MIT license (see LICENSE file)
+ *****************************************************************************/
+
+#ifndef CONSTRAINT_PROFILING_HPP_
+#define CONSTRAINT_PROFILING_HPP_
+
+#include <cstddef>
+#include <map>
+#include <string>
+#include <vector>
+
+namespace libsnark {
+
+extern size_t constraint_profiling_indent;
+
+struct constraint_profiling_entry {
+    size_t indent;
+    std::string annotation;
+    size_t count;
+};
+
+extern std::vector<constraint_profiling_entry> constraint_profiling_table;
+
+#define PROFILE_CONSTRAINTS(pb, annotation)                             \
+    for (size_t _num_constraints_before = pb.num_constraints(), _iter = (++constraint_profiling_indent, 0), _cp_pos = constraint_profiling_table.size(); \
+         _iter == 0;                                                    \
+         constraint_profiling_table.insert(constraint_profiling_table.begin() + _cp_pos, constraint_profiling_entry{--constraint_profiling_indent, annotation, pb.num_constraints() - _num_constraints_before}), \
+         _iter = 1)
+
+size_t PRINT_CONSTRAINT_PROFILING(); // returns # of top level constraints
+
+} // libsnark
+
+#endif // CONSTRAINT_PROFILING_HPP_