FindCustomPython3: allow to find the Python platform in a non-standard directory [MacOS]

cmake/multipython/FindCustomPython3.cmake

@@ -21,8 +21,11 @@ function(find_custom_python3)
 
     set(version_minor_min 8) # minimum Python version = 3.8
 
-    set(_CMAKE_FIND_FRAMEWORK ${CMAKE_FIND_FRAMEWORK})
-    set(CMAKE_FIND_FRAMEWORK NEVER)
+    if(APPLE)
+        # on MacOS, this is needed to find the Python platform in a non-standard directory
+        set(__CMAKE_FIND_FRAMEWORK ${CMAKE_FIND_FRAMEWORK})
+        set(CMAKE_FIND_FRAMEWORK NEVER)
+    endif(APPLE)
 
     # if DEFAULT_PATH is set, the Python platform will be sought in
     # the CMake default paths. This is used for finding the _main_
@@ -36,7 +39,9 @@ function(find_custom_python3)
             PATHS "${__PATHS}" NO_DEFAULT_PATH)
     endif()
 
-    set(CMAKE_FIND_FRAMEWORK ${_CMAKE_FIND_FRAMEWORK})
+    if(APPLE)
+        set(CMAKE_FIND_FRAMEWORK ${__CMAKE_FIND_FRAMEWORK})
+    endif(APPLE)
 
     # store the exported parameters in the internal cache;
     # eg., with pytag 'py39', 'Python3_LIBRARY_DIRS_py39' will be exported, etc.