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/gtests.cpp

@@ -0,0 +1,12 @@
+#include <gtest/gtest.h>
+
+#include "common/profiling.hpp"
+
+int main(int argc, char **argv) {
+  libsnark::inhibit_profiling_info = true;
+  libsnark::inhibit_profiling_counters = true;
+
+  testing::InitGoogleTest(&argc, argv);
+  return RUN_ALL_TESTS();
+}
+