The primary use case behind providing an abstraction layer to enable
alternative Elliptic Curve Point arithmetic implementation, is making
use of cryptographic acceleration hardware if it is present.
To provide thread safety for the hardware accelerator we need a mutex
to guard it.
* gmtime_r is not standard so -std=c99 warns about it
* Anyway we need global mutexes in the threading layer, so better depend only
on that, rather that global mutexes + some _r functions
