As the library approaches 1.0 we should provide a symbol to ask for the version of libsecp256k1
An example such as what is provided by OpenSSL - https://www.openssl.org/docs/man1.1.0/man3/OPENSSL_VERSION_NUMBER.html
Before the 1.0 release candidate series this can use a value of 0.8 or 0.9 to reflect the non-stable API surface