feat: Add option to static linking with openssl

[ikrivosheev]

1. Added YARA_OPENSSL_STATIC env for static linking with openssl
2. Rename env OPENSSL_DIR to YARA_OPENSSL_DIR - remove name conflict with rust-openssl
3. Rename env OPENSSL_LIB_DIR to YARA_OPENSSL_LIB_DIR - remove name conflict with rust-openssl
4. Rename env OPENSSL_INCLUDE_DIR to YARA_OPENSSL_INCLUDE_DIR - remove name conflict with rust-openssl
5. Rename env LIBYARA_STATIC to YARA_STATIC - for uniform with other envs
6. Improve doc

@vthib you added envs for openssl, can you look at the PR? My thought was: remove name conflict with rust-openssl, maybe I am not right and it's ok.

[Hugal31]

LGTM!
Did you consider to use a cargo feature for the YARA_STATIC and YARA_OPENSSL_STATIC? Is that something we want?

[ikrivosheev]

LGTM! Did you consider to use a cargo feature for the YARA_STATIC and YARA_OPENSSL_STATIC? Is that something we want?

Yes! It makes sense! Done)

[vthib]

@ikrivosheev I added them to mimic rust-openssl, so that setting the env can work for both.

I'm not sure why the collision would be a bad thing, unless you want to use different versions of openssl for both.

But splitting them is fine too, just a bit more annoying when both OPENSSL_* and YARA_OPENSSL_* have to be set instead of just the OPENSSL_* one

[ikrivosheev]

@Hugal31 do we want to add support both envs: OPENSSL_* and YARA_OPENSSL_*?