Bugfix
   * Fix build errors when the only enabled elliptic curves are Montgomery
     curves. Raised by signpainter in #941 and by Taiki-San in #1412. This
     also fixes missing declarations reported by Steven Cooreman in #1147.
   * Fix self-test failure when the only enabled short Weierstrass elliptic
     curve is secp192k1. Fixes #2017.
