https://lists.samba.org/archive/rsync-announce/2022/000111.html
--- a/configure.ac
+++ b/configure.ac
@@ -1117,7 +1117,7 @@ else
 fi
 
 AC_CACHE_CHECK([for unsigned char],rsync_cv_SIGNED_CHAR_OK,[
-AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]], [[signed char *s = ""]])],[rsync_cv_SIGNED_CHAR_OK=yes],[rsync_cv_SIGNED_CHAR_OK=no])])
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]], [[signed char *s = (signed char *)""]])],[rsync_cv_SIGNED_CHAR_OK=yes],[rsync_cv_SIGNED_CHAR_OK=no])])
 if test x"$rsync_cv_SIGNED_CHAR_OK" = x"yes"; then
     AC_DEFINE(SIGNED_CHAR_OK, 1, [Define to 1 if "signed char" is a valid type])
 fi
