macOS os.getversion() improvements

[noresources]

What does this PR do?

• Read macOS system version from system property list instead of reading kernel version.
• Add description for latest versions

How does this PR change Premake's behavior?

• Same behavior for previously supported OS versions
• Correct version number for any new version of macOS instead of 0.0
• Maybe a bit slower

Anything else we should know?

• I have kept the previous method as a fallback if the property list is removed in future releases.
• The big switch over kernel version has been replaced by info->minorversion = kern_major - 4;. The relation is correct for all previously supported versions and probably for future ones.
• The description will remain "Mac OS X" for new versions until a manual update of the code.

There is simpler methods to retrieve the OS version but they use private API or Objective-C code.

Add any other context about your changes here.

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
• Minimize the number of commits

[samsinsane]

There is simpler methods to retrieve the OS version but they use private API or Objective-C code.

Is there a simpler Objective-C version that uses public APIs? How much simpler?

@starkos would you object to having a simpler implementation written in a different language? The code is already platform specific, being written in the "platform language" isn't going to make that much of a difference.

[noresources]

@samsinsane Objective-C code relies one Cocoa AppKit framework. It is not simpler, just use standard APIs.
Here is an implementation example

[starkos]

I would not object to using Objective C in the macOS build.

[noresources]

I have made some exeperiments with the Objective-C API and the sw_vers command line tool

1. If I modify the SystemVersion.plist values, both sw_vers and the Objective-C API returns the modified values immediatly. So, the current pull request does exactly what Apple is doing under the hood
2. The Objective-C API is only available on OSX 10.10+ (with a deprecated alternative)
3. To compile Objective-C, Bottstrap.mak and premake4/5.lua have to be changed.

[starkos]

Thanks for doing that research. @samsinsane I'm okay with the current, simpler plist approach, if you have no objections?