On NixOS the session.so php extension is a separate package (and not builtin
in libphp.so). But since the uwsgi php plugin uses the session
mechanisms, we need to link the plugin to that library too.
With this change uwsgipluginpy is hacked to take an additional UWSGICONFIG_PHP_LDFLAGS
environment variable and add it to its LDFLAGS, and then in the nix
expression the UWSGICONFIG_PHP_LDFLAGS is set to point to php.extensions.session
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