CiviCRM doesn't document nor check a requirement

CiviCRM uses set_include_path() to set the include path. However, this call may fail in some circumstances and CiviCRM doesn't check that it succeeds. This happens when the php-module of the apache server is configured with a line starting with

php_admin_value include_path

This setting can not be overridden by set_include_path()! The solution is the change the server configuration so that it uses a line starting with

php_value include_path

instead.

This should be documented and tested for. This causes the automatic installation script for drupal to fail as well, so I had to do a manual installation only to discover that I still got a similar error (this was quite difficult to track down). This should be part of the testing done by the installation script.