Revert "[CMake] Remove overriding CMAKE_OSX_SYSROOT"

CMakeLists.txt

@@ -12,6 +12,32 @@ if(WIN32)
   cmake_policy(SET CMP0091 OLD)
   set(CMAKE_SKIP_TEST_ALL_DEPENDENCY TRUE)
 endif()
+if(CMAKE_HOST_APPLE AND (NOT CMAKE_OSX_SYSROOT OR CMAKE_OSX_SYSROOT STREQUAL ""))
+  # The SYSROOT *must* be set before the project() call
+  # TODO: The following code is *necessary* on a standard MacOS platform (e.g. an Apple MacOS machine), but may not
+  # be as necessary on other MacOS platforms (e.g. Nix). A PR removed the following code https://github.com/root-project/root/pull/19718
+  # and the CI was happy with it, but unfortunately it was working simply because during our CI run jobs something is
+  # setting the SDKROOT environment variable, which implicitly sets the CMAKE_OSX_SYSROOT variable. This was highlighted
+  # by another PR https://github.com/root-project/root/pull/19855. On a standard Apple MacOS system, where the SDKROOT
+  # variable is not set by default, the build of ROOT would fail without the following code. It is unclear what is setting
+  # the SDKROOT variable only during our CI jobs, logging to the same CI machines and launching a build manually shows
+  # the same build error.
+  find_program(XCRUN_EXECUTABLE xcrun)
+  if(EXISTS ${XCRUN_EXECUTABLE})
+    execute_process(COMMAND ${XCRUN_EXECUTABLE} --sdk macosx --show-sdk-path
+      OUTPUT_VARIABLE SDK_PATH
+      ERROR_QUIET
+      OUTPUT_STRIP_TRAILING_WHITESPACE)
+    if(NOT EXISTS "${SDK_PATH}")
+      message(FATAL_ERROR "Could not detect macOS SDK path")
+    endif()
+
+    set(CMAKE_OSX_SYSROOT "${SDK_PATH}" CACHE PATH "SDK path" FORCE)
+  else()
+    message(WARNING "The CMAKE_OSX_SYSROOT variable is not set and the 'xcrun' executable is not available. "
+                    "This build of ROOT might not be able to find the correct MacOS SDK.")
+  endif()
+endif()
 
 if(CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR AND NOT ALLOW_IN_SOURCE)
   message(FATAL_ERROR