Allow to disable reference of AES-256-XTS for Themis iOS

[vixentael]

Currently building Themis iOS with BoringSSL as backend leads to error:

Undefined symbols for architecture x86_64:
  "_EVP_aes_256_xts", referenced from:
      _algid_to_evp in libthemis.a(soter_sym.o)
ld: symbol(s) not found for architecture x86_64

AES-256-XTS implementation is missing in BoringSSL CocoaPod. BoringSSL CocoaPod is rather popular and used in thousands apps, and probably they had a reason to exclude XTS, so I won't suggest them to change their podspec.

However, Themis is not really using XTS mode (see search results), but soter_sym has two references on it (see soter/openssl/soter_sym and soter/boringssl/soter_sym).

I suggest to have a way to exclude references to EVP_aes_256_xts for Themis iOS using #define SOTER_BORINGSSL_DISABLE_XTS.

This change won't affect other platforms, but will be used only in themis.podspec for iOS.

[ignatk]

Should we also add logic in the makefile, which will add SOTER_BORINGSSL_DISABLE_XTS if building Themis on iOS with BoringSSL?

[ignatk]

Please, use consistent comments as the rest of the file.

[vixentael]

what kind of comment style would you prefer? because i don't see any consistency across comments in all soter files.

[ignatk]

Well, at least in this file we have /* ... */ at the top. I generally prefer C-style comments in C files.

[vixentael]

sure, changed.

when i was reading other files, I noticed rather mixed style of comments, but my pleasure is to make it more consistent :)

[vixentael]

Should we also add logic in the makefile, which will add SOTER_BORINGSSL_DISABLE_XTS if building Themis on iOS with BoringSSL?

Themis for iOS is not built via Makefile, so no need to change it.