Binding of libsecp256k1 of Bitcoin ABC implementation

Prerequirement

• CMake v3.5~
  Used for compiling libsecp256k1.
• clang v3.7~ Used for generating bindings.
  More info: https://github.com/rust-lang/rust-bindgen/blob/master/book/src/requirements.md
• svn
  Used for downloading sources of libsecp256k1 from github.

Environment variable

You can customize build step using following environment variables. - BITCOINABCREPOURL
default: https://github.com/Bitcoin-ABC/bitcoin-abc/tags/v0.19.6/
Repository of Bitcoin ABC.
libsecp256k1 sources are downloaded from ${BITCOINABCREPOURL}src/secp256k1 .