Add support for different crypto backends

[rmader]

Hello,
as libomemo uses only quite basic functionality from libgcrypt (afaik) , I'd like to make it work with other backends as well, like nss or openssl.
That would make it easier for many projects to include libomemo (for example Thunderbird, which uses nss as it's only crypto backend).

Would you generally accept patches in this direction? It would increase complexity a bit and definitely needs to get reviewed by experienced people, but I think there are some at my university to help with that.

Cheers at thanks for this project!

[gkdr]

Hi, I think this is already easily possible, as the omemo_crypto_provider is not opaque and the code using this library is supposed to instantiate it with three function pointers. The few times crypto operations are performed in the code, such a struct needs to be passed to the function - there are no internal dependencies. The libomemo_crypto module is just supposed to be an example implementation (which I use in my code).
If you actually have a need for that and create a makefile target that does not depend on gcrypt, sure, I'll accept that pull request. So far, the need just didn't arise.
(In fact, I initially used OpenSSL and switched to gcrypt for license reasons, I guess if you really want to you can dig through the old commits and recover the code and offer an alternative target just as well.)