[3.15] gh-143632: Fix the musl check for PR_SET_VMA_ANON_NAME (GH-159886) (GH-159919)

The guard added by gh-143730 tested $ac_cv_libc, which is never set, so the
check always ran and the build could fail on the struct prctl_mm_map
redefinition it was meant to avoid.  Match on the host triplet instead.
(cherry picked from commit f37e821)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: 3 people

Misc/NEWS.d/next/Build/2026-10-20-17-50-00.gh-issue-143632.PrCtlM.rst

@@ -0,0 +1,3 @@
+Fix the check that disables ``PR_SET_VMA_ANON_NAME`` on musl libc: it tested
+an unset variable, so it never had any effect and the build could fail with
+a redefinition of ``struct prctl_mm_map``.

configure

@@ -5860,15 +5860,17 @@ AC_CHECK_DECLS([UT_NAMESIZE],
                          [Define if you have the 'HAVE_UT_NAMESIZE' constant.])],
               [],
               [@%:@include <utmp.h>])
-# musl libc redefines struct prctl_mm_map and conflicts with linux/prctl.h
-AS_IF([test "$ac_cv_libc" != musl], [
-AC_CHECK_DECLS([PR_SET_VMA_ANON_NAME],
-              [AC_DEFINE([_Py_HAVE_PR_SET_VMA_ANON_NAME], [1],
-                         [Define if you have the 'PR_SET_VMA_ANON_NAME' constant.])],
-              [],
-              [@%:@include <linux/prctl.h>
-               @%:@include <sys/prctl.h>])
-])
+# musl libc redefines struct prctl_mm_map and conflicts with linux/prctl.h.
+# The conftest does not reliably catch this, so skip the check on musl.
+AS_CASE([$host],
+  [*-musl*], [],
+  [AC_CHECK_DECLS([PR_SET_VMA_ANON_NAME],
+                [AC_DEFINE([_Py_HAVE_PR_SET_VMA_ANON_NAME], [1],
+                           [Define if you have the 'PR_SET_VMA_ANON_NAME' constant.])],
+                [],
+                [@%:@include <linux/prctl.h>
+                 @%:@include <sys/prctl.h>])]
+)
 # check for openpty, login_tty, and forkpty
 
 AC_CHECK_FUNCS([openpty], [],