I understand that it isn't a nice idea having code and configuration mixed together and that splitting them would be preferable. However, howm takes less than 1 second to compile on my laptop, once you have a configuration that you like you'll rarely change it and I feel that having the configuration in code makes it easier for hackers to find subtle new ways of customising howm.

A downside that I can see to using a header file for configuration is that a compile is required for installation and configuration. I don't see this as being too much of an issue as I don't plan on making howm available as a binary, so a compiler would be required anyway.