macOS: Check for utimensat and futimens does not consider deployment target

Building CMake using the macOS 10.13 SDK finds utimensat and futimens even if the deployment target is less than 10.13, and older versions do not provide the symbols.

Minimal example with cmake 3.9.0. Using gtest (from our normal build) and a temporary zip file I made with "Compress..." in Finder.

$ cmake -E tar xfz /Developer/SDKs/gtest/gtest-1.7.0.zip
dyld: lazy symbol binding failed: Symbol not found: _futimens
  Referenced from: /usr/local/bin/cmake
  Expected in: /usr/lib/libSystem.B.dylib

dyld: Symbol not found: _futimens
  Referenced from: /usr/local/bin/cmake
  Expected in: /usr/lib/libSystem.B.dylib

Abort trap: 6

$ cmake -E tar xfz temp.zip
dyld: lazy symbol binding failed: Symbol not found: _utimensat
  Referenced from: /usr/local/bin/cmake
  Expected in: /usr/lib/libSystem.B.dylib

dyld: Symbol not found: _utimensat
  Referenced from: /usr/local/bin/cmake
  Expected in: /usr/lib/libSystem.B.dylib

Abort trap: 6