This directory stores modules which require external libraries to compile. For example, m_regex_pcre requires the PCRE libraries. To compile any of these modules first ensure you have the required dependencies (read the online documentation at https://docs.inspircd.org) and then symlink the .cpp file from this directory into the parent directory (src/modules/). Alternatively, use the command: ./configure --enable-extras=m_extra.cpp, which will automatically take care of symlinking the module of that name and any headers it needs from the extras directory. You can also use ./configure --list-extras to see what modules are available, and whether they are enabled or not. You still are responsible for resolving any external dependencies however (ie, PCRE, MySQL, GnuTLS, etc). NOTE: Some modules require you to copy other dependencies too, such as a related header. If you get missing header errors when building these modules, check that the header is not within modules/extra before checking for external dependencies.