[CMake] Library with source in multiple sub-directories

[Mr David McKinnon]

Hi,
  I have a fairly simple question that in my initial attempts I have not
been able solve sufficiently.

  I would like to build a library with the ADD_LIBRARY command from a
set of source files that span multiple sub-directories. I am creating a
list of the source files using the SET command in each of the
sub-directories (SET(source1 ...)) then trying to append all the sources
together in the top-level directory as follows SET(all_source ${source1}
${source2} ...) and using these as follows ADD_LIBRARY(a_library
${all_source}) but the result is that non of the source files from
${source1} etc. appear in the source file list in ${all_source}.

  Is this right approach to this type of library building? Am I missing
something on the lower directory levels that does not permit the source
file information to be passed to the higher levels.

Thanks,
David McKinnon...