[CMake] Preserve file date on copy

[Bill Hoffman]

Yann Renard wrote:
> I have several modules managed with cmake and a script to build all 
> the modules at once. When a simple module gets modified, cmake 
> rebuilds what is needed in it and copies the API files in the include 
> path. Those API files' date get modified at this time, resulting on 
> the rebuild off all modules depending on this first simple module. My 
> wish would be to avoid date change at copy time to free some time 
> during build process. (copies are made thanks to cmake command mode 
> calls)
The best thing to do would be a copy if different.   If the files really 
do change, then you would want a rebuild.  Both configure file and -E 
copy_if_different will work.

-Bill