handle BoringSSL as crypto lib in yara-sys

[vthib]

YARA handles the use of BoringSSL as the crypto lib, with a caveat: BORINGSSL must be defined. This is so the pe.signatures handling can be removed, while keeping all the rest of the crypto handling, for example the hash module.

To handle that, a new cryptolib type is added, which acts as openssl, but adds this define.

[vthib]

@vthib hello! Thank you for the PR! Can you add some information about it: https://github.com/Hugal31/yara-rust/blob/master/yara-sys/README.md ?

Yes of course, sorry about this, done.

[Hugal31]

LGTM, thanks!