Now we must edit src/apache2/mod_suphp.c because otherwise you'll get an error like this one

Wrong approach. The developer of SuPHP intentionally didn't allow the SuPHP_AddHandler directive in some blocks, like the global Apache configuration or plain VirtualHost-Blocks. Instead of configuring your Apache httpd correctly you patch the SuPHP source. Maybe you want to read about the Location- and Directory-directives in the Apache manual?