Add support for the "header_only" option

[Croydon]

CCI has a special handling for recipes with a header_only option and it would be great to have that supported in CPT too

If header_only is per default False, CPT should add another build where it uses the the other default options but switches header_only to True

If header_only is per default True, CPT should add this one header-only default build and then add all the regular combinations with header_only set to False