macOS: Avoid relying on sw_vers when cross-compiling from another OS

We have a few code paths needed only to support builds hosted on
macOS 10.4.  Update the conditions to work when cross-compiling
to macOS from another platform.

Fixes: #26814

Modules/Platform/Darwin-Initialize.cmake

@@ -15,9 +15,11 @@ else()
   set(OSX_DEVELOPER_ROOT "")
 endif()
 
-execute_process(COMMAND sw_vers -productVersion
-  OUTPUT_VARIABLE CURRENT_OSX_VERSION
-  OUTPUT_STRIP_TRAILING_WHITESPACE)
+if(NOT CMAKE_CROSSCOMPILING)
+  execute_process(COMMAND sw_vers -productVersion
+    OUTPUT_VARIABLE _CMAKE_HOST_OSX_VERSION
+    OUTPUT_STRIP_TRAILING_WHITESPACE)
+endif()
 
 # Save CMAKE_OSX_ARCHITECTURES from the environment.
 set(CMAKE_OSX_ARCHITECTURES "$ENV{CMAKE_OSX_ARCHITECTURES}" CACHE STRING
@@ -47,12 +49,6 @@ endif()
 # Platform/Apple-${CMAKE_CXX_COMPILER_ID}-<LANG>
 set(CMAKE_EFFECTIVE_SYSTEM_NAME "Apple")
 
-#----------------------------------------------------------------------------
-# _CURRENT_OSX_VERSION - as a two-component string: 10.5, 10.6, ...
-#
-string(REGEX REPLACE "^([0-9]+\\.[0-9]+).*$" "\\1"
-  _CURRENT_OSX_VERSION "${CURRENT_OSX_VERSION}")
-
 #----------------------------------------------------------------------------
 # CMAKE_OSX_DEPLOYMENT_TARGET
 

Modules/Platform/Darwin.cmake

@@ -47,7 +47,7 @@ set(CMAKE_APPLE_IMPORT_FILE_PREFIX "lib")
 set(CMAKE_APPLE_IMPORT_FILE_SUFFIX ".tbd")
 set(CMAKE_MODULE_EXISTS 1)
 set(CMAKE_DL_LIBS "")
-if(NOT "${_CURRENT_OSX_VERSION}" VERSION_LESS "10.5")
+if(NOT (DEFINED _CMAKE_HOST_OSX_VERSION AND _CMAKE_HOST_OSX_VERSION VERSION_LESS "10.5"))
   set(CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG "-Wl,-rpath,")
 endif()
 
@@ -80,7 +80,7 @@ endif()
 # Enable shared library versioning.
 set(CMAKE_SHARED_LIBRARY_SONAME_C_FLAG "-install_name")
 
-if("${_CURRENT_OSX_VERSION}" VERSION_LESS "10.5")
+if(DEFINED _CMAKE_HOST_OSX_VERSION AND _CMAKE_HOST_OSX_VERSION VERSION_LESS "10.5")
   # Need to list dependent shared libraries on link line.  When building
   # with -isysroot (for universal binaries), the linker always looks for
   # dependent libraries under the sysroot.  Listing them on the link
@@ -157,7 +157,8 @@ if(_CMAKE_OSX_SYSROOT_PATH)
   list(APPEND CMAKE_PLATFORM_IMPLICIT_LINK_FRAMEWORK_DIRECTORIES
     /System/Library/Frameworks)
 endif()
-if("${_CURRENT_OSX_VERSION}" VERSION_LESS "10.5")
+
+if(DEFINED _CMAKE_HOST_OSX_VERSION AND _CMAKE_HOST_OSX_VERSION VERSION_LESS "10.5")
   # Older OS X tools had more implicit paths.
   list(APPEND CMAKE_PLATFORM_IMPLICIT_LINK_FRAMEWORK_DIRECTORIES
     ${_CMAKE_OSX_SYSROOT_PATH}/Network/Library/Frameworks)