[CMake] set_property on sources causing extra files to be rebuilt

[Daniel Nelson]

It looks like the projects flags.make is updated every time any compile flag or 
definition changes, and every object file in the project depends on flags.make.  
In the case of properties set using set_property() the only part of the 
flags.make that changes is the comment, so maybe it would be best to put these 
comments into a different file that is not a dependency?