Make EC25519 implementation optional for static library

[david-komarek]

Description

curve25519-donna-c64.c/curve25519-donna.c is built even for static libraries and this could lead to multiple implementation of ec25519 in binary.

Steps to reproduce

• build noise-c as static library

Actual result: ec25519 implementation is part of static library.
Expected result: user should decide whether he/she wants the ec25519 implementation to be part of static library.