fix: add flag to opt-in to new (cross-compile) behavior

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

tools/FindPythonLibsNew.cmake

@@ -95,24 +95,9 @@ if(NOT PythonLibsNew_FIND_VERSION)
   set(PythonLibsNew_FIND_VERSION "3.6")
 endif()
 
-# Save variables that get set by PythonInterp
-macro(_PYBIND11_PUSH_IF_DEFINED name)
-  if(DEFINED "${name}")
-    set("_PYBIND11_ORIG_${name}" "${${name}}")
-  endif()
-endmacro()
-
-_pybind11_push_if_defined(PYTHON_INCLUDE_DIR)
-_pybind11_push_if_defined(PYTHON_MODULE_EXTENSION)
-_pybind11_push_if_defined(PYTHON_IS_DEBUG)
-
 find_package(PythonInterp ${PythonLibsNew_FIND_VERSION} ${_pythonlibs_required}
              ${_pythonlibs_quiet})
 
-unset(PYTHON_INCLUDE_DIR)
-unset(PYTHON_MODULE_EXTENSION)
-unset(PYTHON_IS_DEBUG)
-
 if(NOT PYTHONINTERP_FOUND)
   set(PYTHONLIBS_FOUND FALSE)
   set(PythonLibsNew_FOUND FALSE)
@@ -166,11 +151,13 @@ if(NOT _PYTHON_SUCCESS MATCHES 0)
   return()
 endif()
 
+option(
+  PYBIND11_PYTHONLIBS_OVERRWRITE
+  "Overwrite cached values read from Python library (classic search). Turn off if cross-compiling and manually setting these values."
+  ON)
 # Can manually set values when cross-compiling
 macro(_PYBIND11_GET_IF_UNDEF lst index name)
-  if(DEFINED "_PYBIND11_ORIG_${name}")
-    set("${name}" "${_PYBIND11_ORIG_${name}}")
-  elseif(NOT DEFINED "${name}")
+  if(PYBIND11_PYTHONLIBS_OVERRWRITE OR NOT DEFINED "${name}")
     list(GET "${lst}" "${index}" "${name}")
   endif()
 endmacro()