Thanks Chris,
set_source_files_properties(my_text_file.txt PROPERTIES HEADER_FILE_ONLY TRUE)
That worked great.
As for the copy, I changed the command to:
add_custom_command(
TARGET ${PROJECT_NAME} POST_BUILD
COMMAND ${CMAKE_COMMAND} -E copy
${CMAKE_CURRENT_SOURCE_DIR}/filenames_to_hash.txt
${CMAKE_CURRENT_BINARY_DIR})
and it worked, sorta. It put it in one level down from where the executable was put. So instead of build/project/Debug/, it put it in build/project/. What should I be making the target directory?