[depends][darwin] cleanup, fix variable order & build / host mix up

[Rechi]

Description

• remove handling for unsupported Xcode versions
• reorder use_sdk_path (assign it before using it)
• -mmacosx-version-min should be in native cflags for all build on osx, no matter what the target platform is
• same is true for ac_cv_* settings in native config.site

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the Code guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the CONTRIBUTING document
• I have added tests to cover my change
• All new and existing tests passed

[Memphiz]

Looks good in general. Jenkins doesn’t seem to like something about it though (only on phone - can’t check the compile output).
This definitely changes some Behavior for libffi - but I guess those libffi hacks are not needed anymore - same for GMP
Approved - once Jenkins builds it...

[Rechi]

@Memphiz jenkins succeeded now, maybe you want to look at the changes again.

[Memphiz]

Mhh OSX-64 failed?

[Rechi]

Test suite only (TestEvent.GroupTimedWait), but will do another jenkins run.
Apart from that good to get merged?

[Memphiz]

Appart from that - fine by me - thx :)

[hudokkow]

Nice work! iphoneos11.3_arm64 build was giving me problems before this with default sdk.

Tested this PR on 10.13.4 (x64, x32 and iOS arm64 clean builds). Apart from the usual shenanigans of having to run make a few times to build everything successfully, all seems to work as intended.

Going to do a round of Kodi builds to test the rest. 🤞

Thanks!