secp256k1: add libtool as a dependency
secp256k1: Add build-side changes for libsecp256k1

Note: This is added to our existing automake targets rather than as a
libtool-style lib. The switch to libtool-style targets can come later if it
proves to not add any complications.
libsecp256k1 integration
Merge pull request #4312

fda3fed libsecp256k1 integration (Pieter Wuille)
5566826 secp256k1: Add build-side changes for libsecp256k1 (Cory Fields)
b150b09 secp256k1: add libtool as a dependency (Cory Fields)