[3.12] gh-120586: Fix several "unused function" warnings in posixmodule.c (GH-120588) (#120617)

gh-120586: Fix several "unused function" warnings in `posixmodule.c` (GH-120588)
(cherry picked from commit 3df2022)

Co-authored-by: Nikita Sobolev <[email protected]>

Author: miss-islington

Modules/posixmodule.c

@@ -7579,6 +7579,7 @@ os_register_at_fork_impl(PyObject *module, PyObject *before,
 }
 #endif /* HAVE_FORK */
 
+#if defined(HAVE_FORK1) || defined(HAVE_FORKPTY) || defined(HAVE_FORK)
 // Common code to raise a warning if we detect there is more than one thread
 // running in the process. Best effort, silent if unable to count threads.
 // Constraint: Quick. Never overcounts. Never leaves an error set.
@@ -7677,6 +7678,7 @@ static void warn_about_fork_with_threads(const char* name) {
         PyErr_Clear();
     }
 }
+#endif  // HAVE_FORK1 || HAVE_FORKPTY || HAVE_FORK
 
 #ifdef HAVE_FORK1
 /*[clinic input]
@@ -11783,6 +11785,7 @@ os_mknod_impl(PyObject *module, path_t *path, int mode, dev_t device,
 #endif /* defined(HAVE_MKNOD) && defined(HAVE_MAKEDEV) */
 
 
+#ifdef HAVE_DEVICE_MACROS
 static PyObject *
 major_minor_conv(unsigned int value)
 {
@@ -11805,7 +11808,6 @@ major_minor_check(dev_t value)
     return (dev_t)(unsigned int)value == value;
 }
 
-#ifdef HAVE_DEVICE_MACROS
 /*[clinic input]
 os.major