[CMake] Set CMAKE_OSX_SYSROOT only when the host system is Apple macOS

Set `CMAKE_OSX_SYSROOT` only when the host system is Apple macO, because
otherwise we don't need it and the `xcrun` executable that is used to
figure out this variable is not available.

Like this, we avoid configuration failues when building ROOT is built
for Darwin in general.

This mimics what was done in CMake before version 4.0:
https://gitlab.kitware.com/search?search=xcrun&nav_source=navbar&project_id=541&group_id=415&search_code=true&repository_ref=v3.31.6

Follows up on 6bd0dba and ebc38bd.

Author: guitargeek

CMakeLists.txt

@@ -12,7 +12,7 @@ if(WIN32)
   cmake_policy(SET CMP0091 OLD)
   set(CMAKE_SKIP_TEST_ALL_DEPENDENCY TRUE)
 endif()
-if(APPLE AND (NOT CMAKE_OSX_SYSROOT OR CMAKE_OSX_SYSROOT STREQUAL ""))
+if(CMAKE_HOST_APPLE AND (NOT CMAKE_OSX_SYSROOT OR CMAKE_OSX_SYSROOT STREQUAL ""))
   # The SYSROOT *must* be set before the project() call
   execute_process(COMMAND xcrun --sdk macosx --show-sdk-path
     OUTPUT_VARIABLE SDK_PATH