BUILD: ssl: add ECDSA_SIG_set0() for openssl < 1.1 or libressl < 2.7

BUILD: ssl: add ECDSA_SIG_set0() for openssl < 1.1 or libressl < 2.7

Commit 5a8f02ae6 ("BUG/MEDIUM: jwt: Properly process ecdsa signatures
(concatenated R and S params)") makes use of ECDSA_SIG_set0() which only
appeared in openssl-1.1.0 and libressl 2.7, and breaks the build before.
Let's just do what it minimally does (only assigns the two fields to the
destination).

This will need to be backported where the commit above is, likely 2.5.