fix(cmake): allow forcing old FindPython (#5042)

tools/pybind11Common.cmake

@@ -173,12 +173,16 @@ endif()
 # Check to see which Python mode we are in, new, old, or no python
 if(PYBIND11_NOPYTHON)
   set(_pybind11_nopython ON)
+  # We won't use new FindPython if PYBIND11_FINDPYTHON is defined and falselike
+  # Otherwise, we use if FindPythonLibs is missing or if FindPython was already used
 elseif(
-  _pybind11_missing_old_python STREQUAL "NEW"
-  OR PYBIND11_FINDPYTHON
-  OR Python_FOUND
-  OR Python2_FOUND
-  OR Python3_FOUND)
+  (NOT DEFINED PYBIND11_FINDPYTHON OR PYBIND11_FINDPYTHON)
+  AND (_pybind11_missing_old_python STREQUAL "NEW"
+       OR PYBIND11_FINDPYTHON
+       OR Python_FOUND
+       OR Python3_FOUND
+      ))
+
   # New mode
   include("${CMAKE_CURRENT_LIST_DIR}/pybind11NewTools.cmake")