53992: check for declaration of fpurge() instead of link-ability

musl-libc has fpurge() but does not declare it in stdio.h. zsh does not
need fpurge() if musl is in use.

Author: floppym

ChangeLog

@@ -1,3 +1,9 @@
+2025-10-21  Jun-ichi Takimoto  <[email protected]>
+
+	* Mike Gilbert: 53992: Src/utils.c, configure.ac: check for
+	declaration of fpurge() instead of its link-ability (as a
+	workaround of the problem of musl-libc)
+
 2025-10-02  Oliver Kiddle  <[email protected]>
 
 	* 53985: Completion/Unix/Command/_git: update to 2.51

Src/utils.c

@@ -2022,7 +2022,7 @@ redup(int x, int y)
 {
     int ret = y;
 
-#ifdef HAVE_FPURGE
+#if HAVE_DECL_FPURGE
     /* Make sure buffers are cleared when changing descriptor for a
      * FILE object.  No fflush() here because the only way anything
      * can legitimately be left in the buffer is when an error has

configure.ac

@@ -1250,7 +1250,7 @@ AC_CHECK_FUNCS(strftime strptime mktime timelocal \
 	       select poll \
 	       readlink faccessx fchdir ftruncate \
 	       fstat lstat lchown fchown fchmod \
-	       fpurge fseeko ftello \
+	       fseeko ftello \
 	       mkfifo _mktemp mkstemp \
 	       waitpid wait3 \
 	       sigqueue \
@@ -1292,6 +1292,9 @@ AC_CHECK_FUNCS(strftime strptime mktime timelocal \
 	       setutxent getutxent endutxent getutent)
 AC_FUNC_STRCOLL
 
+# fpurge exists on musl, but is undeclared in stdio.h and isn't actually required
+AC_CHECK_DECLS([fpurge])
+
 # isinf() and isnan() can exist as either functions or macros.
 AH_TEMPLATE([HAVE_ISINF],
   [Define to 1 if you have the `isinf' macro or function.])