Preserve parent Apple deployment target

[tryk016]

Tempest currently resets CMAKE_OSX_DEPLOYMENT_TARGET to 12.0 even when a parent project or the command line already selected another value.

For example, OpenGothic sets iOS 15.0, but Tempest sources are still compiled with a 12.0 minimum.

Keep 12.0 as the standalone default, but only apply it when no deployment target has been configured.

Validation:

• current master with a 15.0 target generates 12.0 compiler settings for iPhoneOS, the iOS Simulator, and macOS
• the patched version preserves 15.0 for all three Apple targets
• fresh iPhoneOS, iOS Simulator, and macOS builds complete successfully
• without an explicit target, the default remains 12.0
• fork CI passes on MSVC, MinGW, Ubuntu, and macOS

[Try]

Merged, thanks!