Depends on libsecp2561k1 being installed on the host

[afk11]

We should be able to build against a copy of libsecp256k1 in git, to avoid the constant breaks in upstream software when libsecp25k1 upgrades. .

I think there are examples of this out there (zip in php-src, for example) but config.m4 scripts are horrible to digest.

Any extension developers who can help, please do get in touch.