correctly support Py_HASH_EXTERNAL for Unix platforms

Author: picnixz

Doc/using/configure.rst

@@ -954,13 +954,14 @@ Libraries options
 Security Options
 ----------------
 
-.. option:: --with-hash-algorithm=[fnv|siphash13|siphash24]
+.. option:: --with-hash-algorithm=[fnv|siphash13|siphash24|external]
 
    Select hash algorithm for use in ``Python/pyhash.c``:
 
    * ``siphash13`` (default);
    * ``siphash24``;
-   * ``fnv``.
+   * ``fnv``;
+   * ``external``.
 
    .. versionadded:: 3.4
 

configure

@@ -3966,12 +3966,15 @@ dnl quadrigraphs "@<:@" and "@:>@" produce "[" and "]" in the output
 AC_ARG_WITH(
   [hash_algorithm],
   [AS_HELP_STRING(
-    [--with-hash-algorithm=@<:@fnv|siphash13|siphash24@:>@],
+    [--with-hash-algorithm=@<:@fnv|siphash13|siphash24|external@:>@],
     [select hash algorithm for use in Python/pyhash.c (default is SipHash13)]
   )],
 [
 AC_MSG_RESULT([$withval])
 case "$withval" in
+    external)
+        AC_DEFINE([Py_HASH_ALGORITHM], [0])
+        ;;
     siphash13)
         AC_DEFINE([Py_HASH_ALGORITHM], [3])
         ;;