patch 9.0.1971: macOS: FEAT_SOUND guard too restrictive

patch 9.0.1971: macOS: FEAT_SOUND guard too restrictive

Problem:  macOS: FEAT_SOUND guard too restrictive
Solution: check for older macOS support properly

Fix macOS FEAT_SOUND guards to be less restrictive

This allows +sound to work on older macOS platforms again. The +sound
implementation uses APIs available in 10.6, but the code itself uses
generics with type parameters which was only added in Xcode 7 / clang 7,
which was released for macOS 10.11. This means as long as Vim is
compiled under 10.11+, and using a deployment target >= 10.6, the
feature will work.

closes: #13251

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>