Ignoring CMAKE_OSX_SYSROOT because the directory does not exist.

After upgrading to Xcode 11, cmake 3.15.3 fails to set default CMAKE_OSX_SYSROOT value, here is the warning message:

CMake Warning at /usr/local/Cellar/cmake/3.15.3/share/cmake/Modules/Platform/Darwin-Initialize.cmake:131 (message):
  Ignoring CMAKE_OSX_SYSROOT value:

   /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk

  because the directory does not exist.
Call Stack (most recent call first):
  /usr/local/Cellar/cmake/3.15.3/share/cmake/Modules/CMakeSystemSpecificInitialize.cmake:21 (include)
  CMakeLists.txt:7 (project)

There is no MacOSX10.14.sdk any more:

% ls /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs
total 0
drwxr-xr-x  5 root  wheel   160B Sep 21 07:01 .
drwxr-xr-x  6 root  wheel   192B Sep 11 19:47 ..
drwxr-xr-x  4 root  wheel   128B Aug 30 14:47 DriverKit19.0.sdk
drwxr-xr-x  8 root  wheel   256B Aug 30 14:47 MacOSX.sdk
lrwxr-xr-x  1 root  wheel    10B Sep 21 06:18 MacOSX10.15.sdk -> MacOSX.sdk

Since MacOSX10.15.sdk is just a symbolic link to MacOSX.sdk, maybe we can set default to /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk

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