component/postfix: disable build on Linux >= 5

This old tag is maintained, so that we can run run ERP5 upgrade test.

On new test nodes, running linux 5, postfix refuses to compile with an error
message like this:

    make -f Makefile.in MAKELEVEL= Makefiles
    (echo "# Do not edit -- this file documents how Postfix was built for your machine."; /bin/sh makedefs) >makedefs.tmp
    ATTENTION:
    ATTENTION: Unknown system type: Linux 5.10.0-0.bpo.4-amd64
    ATTENTION:
    make: *** [Makefile.in:22: Makefiles] Error 1
    make: *** [Makefile:21: makefiles] Error 2

We don't really need postfix for these tests, so skip postfix compilation.

component/postfix/buildout.cfg

@@ -24,3 +24,9 @@ configure-options = makefiles CCARGS='-DUSE_SASL_AUTH -DUSE_CYRUS_SASL -DUSE_TLS
 make-targets = non-interactive-package install_root=${:location}
 environment =
   PATH=${patch:location}/bin:%(PATH)s
+
+# XXX we don't strongly depend on postfix and this version of postfix refuses
+# to compile on Linux >= 5
+[postfix:platform.system() == 'Linux' and platform.release() >= '5']
+recipe =
+location = POSTFIX_NOT_INSTALLED_UNSUPPORTED_OS