build: require POSIX strdup()

Stop detecting this function and drop the local fallback.

Let us know if this update is causing an issue.

Notes:
- on Windows `_strdup()` is required instead.
- `strdup()/_strdup()` were required before this patch to build one of
  the examples: `block_ip`.
- `strdup()/_strdup()` were required in 8.18.0 and earlier to build
  tests.

Closes #20505

m4/curl-functions.m4

@@ -3935,92 +3935,6 @@ AC_DEFUN([CURL_CHECK_FUNC_STRCMPI], [
 ])
 
 
-dnl CURL_CHECK_FUNC_STRDUP
-dnl -------------------------------------------------
-dnl Verify if strdup is available, prototyped, and
-dnl can be compiled. If all of these are true, and
-dnl usage has not been previously disallowed with
-dnl shell variable curl_disallow_strdup, then
-dnl HAVE_STRDUP will be defined.
-
-AC_DEFUN([CURL_CHECK_FUNC_STRDUP], [
-  AC_REQUIRE([CURL_INCLUDES_STRING])dnl
-  AC_REQUIRE([CURL_INCLUDES_STDLIB])dnl
-  #
-  tst_links_strdup="unknown"
-  tst_proto_strdup="unknown"
-  tst_compi_strdup="unknown"
-  tst_allow_strdup="unknown"
-  #
-  AC_MSG_CHECKING([if strdup can be linked])
-  AC_LINK_IFELSE([
-    AC_LANG_FUNC_LINK_TRY([strdup])
-  ],[
-    AC_MSG_RESULT([yes])
-    tst_links_strdup="yes"
-  ],[
-    AC_MSG_RESULT([no])
-    tst_links_strdup="no"
-  ])
-  #
-  if test "$tst_links_strdup" = "yes"; then
-    AC_MSG_CHECKING([if strdup is prototyped])
-    AC_EGREP_CPP([strdup],[
-      $curl_includes_string
-    ],[
-      AC_MSG_RESULT([yes])
-      tst_proto_strdup="yes"
-    ],[
-      AC_MSG_RESULT([no])
-      tst_proto_strdup="no"
-    ])
-  fi
-  #
-  if test "$tst_proto_strdup" = "yes"; then
-    AC_MSG_CHECKING([if strdup is compilable])
-    AC_COMPILE_IFELSE([
-      AC_LANG_PROGRAM([[
-        $curl_includes_string
-        $curl_includes_stdlib
-      ]],[[
-        free(strdup(""));
-      ]])
-    ],[
-      AC_MSG_RESULT([yes])
-      tst_compi_strdup="yes"
-    ],[
-      AC_MSG_RESULT([no])
-      tst_compi_strdup="no"
-    ])
-  fi
-  #
-  if test "$tst_compi_strdup" = "yes"; then
-    AC_MSG_CHECKING([if strdup usage allowed])
-    if test "x$curl_disallow_strdup" != "xyes"; then
-      AC_MSG_RESULT([yes])
-      tst_allow_strdup="yes"
-    else
-      AC_MSG_RESULT([no])
-      tst_allow_strdup="no"
-    fi
-  fi
-  #
-  AC_MSG_CHECKING([if strdup might be used])
-  if test "$tst_links_strdup" = "yes" &&
-     test "$tst_proto_strdup" = "yes" &&
-     test "$tst_compi_strdup" = "yes" &&
-     test "$tst_allow_strdup" = "yes"; then
-    AC_MSG_RESULT([yes])
-    AC_DEFINE_UNQUOTED(HAVE_STRDUP, 1,
-      [Define to 1 if you have the strdup function.])
-    curl_cv_func_strdup="yes"
-  else
-    AC_MSG_RESULT([no])
-    curl_cv_func_strdup="no"
-  fi
-])
-
-
 dnl CURL_CHECK_FUNC_STRERROR_R
 dnl -------------------------------------------------
 dnl Verify if strerror_r is available, prototyped, can be compiled and