opensmtpd: support cross compile (#170379)

* opensmtpd: support cross compile

* add comment to remember the linked PR

Co-authored-by: Léo Gaspard <leo@gaspard.io>

pkgs/servers/mail/opensmtpd/cross_fix.diff

@@ -0,0 +1,13 @@
+diff --git a/configure.ac b/configure.ac
+index c215f3bf..f5aa25d8 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -67,7 +67,7 @@ AC_C_BIGENDIAN
+ AC_PROG_CPP
+ AC_PROG_INSTALL
+ AC_PROG_LIBTOOL
+-AC_PATH_PROG([AR], [ar])
++AC_PATH_TOOL([AR], [ar])
+ AC_PATH_PROG([CAT], [cat])
+ AC_PATH_PROG([CHMOD], [chmod])
+ AC_PATH_PROG([CHOWN], [chown])

pkgs/servers/mail/opensmtpd/default.nix

@@ -16,6 +16,7 @@ stdenv.mkDerivation rec {
 
   patches = [
     ./proc_path.diff # TODO: upstream to OpenSMTPD, see https://github.com/NixOS/nixpkgs/issues/54045
+    ./cross_fix.diff # TODO: remove when https://github.com/OpenSMTPD/OpenSMTPD/pull/1177 will have made it into a release
   ];
 
   # See https://github.com/OpenSMTPD/OpenSMTPD/issues/885 for the `sh bootstrap`