patch: add environment variable override

[colin@xxxxxxxxxx]

The patch below adds SetEnvIf environment variable support that can be used 
to disable mod_xslt. Effectively, this can be used to mimics the 
functionality of XSLTAddRule but can also be used for dynamically assigned 
xslt. With this patch you can AddOutputFilter and then disable it based on 
any criteria necessary. This allows for more customized control over the 
execution of the mod_xslt module.

For example:
<Location ~ "\.php|/$|^[^.]*$">
        SetOutputFilter mod-xslt
</Location>
SetEnvIf Request_URI /Rss.php no-xslt

This will enable the OutputFilter for all .php files (incl welcome pages) but 
disable it for the rss.php. While this is a primitive example, my scenario 
required adjustment based on Headers and parameters which is much more 
complicated for <Location> matches.

Also this patch also moves the removal of the Content-Length to after the 
quick exit block. This is important so that pipelining is preserved even if 
the mod_xslt is dynamically removed from the chain.

--- modxslt-2005072700/sapi/apache2/modxslt.c.orig   2005-07-27 
04:27:57.000000000 -0600
+++ modxslt-2005072700/sapi/apache2/modxslt.c        2007-01-24 
15:03:35.000000000 -0600
@@ -138,7 +138,8 @@
   const char * forcestyle, * defaultstyle;
   void * get;

-  apr_table_unset(f->r->headers_out, "Content-Length");
+  if (apr_table_get(f->r->subprocess_env, "no-xslt"))
+    return ap_pass_brigade(f->next, brigade);

   if(f->r->header_only)
     return ap_pass_brigade(f->next, brigade);
@@ -148,6 +149,8 @@
   if(f->r->status != HTTP_OK)
     return ap_pass_brigade(f->next, brigade);

+  apr_table_unset(f->r->headers_out, "Content-Length");
+
   data=(apr_bucket_brigade *)f->ctx;
   ap_save_brigade(f, &data, &brigade, f->r->pool);
   f->ctx=data;