Add a build flag to disable building data for cross compilation

[alex-the-man]

Hello, I'm using OpenCC on iOS.

When OpenCC is cross compiled, the build fails as https://github.com/BYVoid/OpenCC/blob/master/data/CMakeLists.txt#L139 executes the cross compiled opencc_dict (targeting arm64) on the host system (x86_64).

Do you mind if I add a build flag BUILD_DATA following https://github.com/BYVoid/OpenCC/blob/master/CMakeLists.txt#L26 to let us disable data/dict generation during cross compilation?

If you like the idea, I could send a PR.