Invalid command 'FastCgiServer', perhaps misspelled or defined by a module not included in the server configuration

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When starting or reloading the apache server, you may get this error:

AH00526: Syntax error on line 4 of /etc/apache2/vhosts.d/cgi-site.conf:
Invalid command 'FastCGIConfig', perhaps misspelled or defined by a module not included in the server configuration

or:
AH00526: Syntax error on line 10 of /etc/apache2/vhosts.d/cgi-site.conf:
Invalid command 'FastCgiServer', perhaps misspelled or defined by a module not included in the server configuration

Cause:
FastCgiServer is defined by mod_fastcgi.
The problem is that mod_fastcgi has not been maintained for a long time and most if not all linux distributions no longer ship a package for it. The source code is still available, though, and can fairly easily be compiled and installed manually. See the main FastCGI article.
The other solution is to use one or the other CGI alternatives, and reconfigure your apache vhost file accordingly.