[3.15] gh-75929: Fix configure function checks under -Wall -Werror (GH-161343) (#161371)

Use `(void)$1` in `PY_CHECK_FUNC` instead of declaring an unused
variable. The unused-variable warning caused configure to report
declared functions such as `fchdir` as missing.
(cherry picked from commit 341f80e)

Author: vasiliyk

Misc/NEWS.d/next/Build/2026-11-04-14-37-13.gh-issue-75929.nELdwE.rst

@@ -0,0 +1,2 @@
+Fix a ``configure`` probe that could omit functions such
+as :func:`os.fchdir` from builds using ``-Wall -Werror``.

configure

@@ -61,7 +61,7 @@ AC_DEFUN([PY_CHECK_FUNC],
     [for $1],
     [py_var],
     [AC_COMPILE_IFELSE(
-      [AC_LANG_PROGRAM([$2], [void *x=$1])],
+      [AC_LANG_PROGRAM([$2], [(void)$1])],
       [AS_VAR_SET([py_var], [yes])],
       [AS_VAR_SET([py_var], [no])])]
   )