Support proper minimum version instead of fishing the latest version

[teras]

The current scheme of iOS version has a few problems:

• It compiles as minimum version the latest version
• There is no way to define the minimum version
• The newest SDK supports only 64 bit mode, since 32 bit was dropped; so an older SDK version is required
• It tries to guess the latest version from a filename scheme, which is fragile and changes in any new SDK version. Actually this is not even necessary since there is a version-less location of the latest SDK.

Please consider this pull request as a base to discuss how these issues could be solved

[dicej]

Thanks for the PR. This looks fine to me. I haven't done any iOS development for a long time, so I don't have any specific opinions on the topic, and I agree that the old way was fragile.

Shall I merge this now, or is there more to discuss and/or change?

[teras]

It works for me :) So if there is no objection, I'd suggest to merge it. If more issues arise, I'll be happy to have a look at it.