Use PYBIND11_CROSSCOMPILING instead of CMAKE_CROSSCOMPILING

tools/pybind11Common.cmake

@@ -42,6 +42,23 @@ set(pybind11_INCLUDE_DIRS
     "${pybind11_INCLUDE_DIR}"
     CACHE INTERNAL "Include directory for pybind11 (Python not requested)")
 
+# Cross-compilation support
+if(NOT DEFINED PYBIND11_CROSSCOMPILING)
+  if(CMAKE_CROSSCOMPILING)
+    message(
+      WARNING
+        "A behavior change for cross-compilation is planned for future versions "
+        "of pybind11. To opt in to the improved cross-compilation logic now and "
+        "to silence this warning, use 'set(PYBIND11_CROSSCOMPILING ${CMAKE_CROSSCOMPILING})'.")
+  endif()
+  set(PYBIND11_CROSSCOMPILING
+      Off
+      CACHE
+        BOOL
+        "Enable cross-compilation mode and avoid using the build machine's Python interpreter to infer properties about the host machine."
+  )
+endif()
+
 # --------------------- Shared targets ----------------------------
 
 # Build an interface library target:
@@ -195,7 +212,7 @@ endif()
 
 # --------------------- pybind11_find_import -------------------------------
 
-if(NOT _pybind11_nopython AND NOT CMAKE_CROSSCOMPILING)
+if(NOT _pybind11_nopython AND NOT PYBIND11_CROSSCOMPILING)
   # Check to see if modules are importable. Use REQUIRED to force an error if
   # one of the modules is not found. <package_name>_FOUND will be set if the
   # package was found (underscores replace dashes if present). QUIET will hide

tools/pybind11NewTools.cmake

@@ -33,7 +33,7 @@ if(NOT Python_FOUND AND NOT Python3_FOUND)
   endif()
 
   # Interpreter should not be found when cross-compiling
-  if(CMAKE_CROSSCOMPILING)
+  if(PYBIND11_CROSSCOMPILING)
     set(_pybind11_interp_component "")
   else()
     set(_pybind11_interp_component Interpreter)
@@ -110,7 +110,7 @@ if(PYBIND11_MASTER_PROJECT)
   endif()
 endif()
 
-if(NOT CMAKE_CROSSCOMPILING)
+if(NOT PYBIND11_CROSSCOMPILING)
   # If a user finds Python, they may forget to include the Interpreter component
   # and the following two steps require it. It is highly recommended by CMake
   # when finding development libraries anyway, so we will require it.