[secp256k1] Set --enable-jni to no by default instead of auto

Summary: This is backport of PR472 by Karl-Johan Alm.

Test Plan:

make check

Reviewers: #bitcoin_abc, schancel, jasonbcox

Reviewed By: #bitcoin_abc, schancel, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1256