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.
Previously libsnark would try to build libgtest if /usr/src/gtest
exists on the build machine. This caused issues because the
version of libgtest in /usr/src isn't necessarily the same as
the one that the rest of zcash's gtests were built with.
