[Cmake] INSTALL_FILES installs unconditionally

[Brad King]

Iker Arizmendi wrote:
> The argument in favor would be that it avoids
> unnecessary builds of other sources that rely
> on the installed files.

It should not be too hard to modify the installation process to do 
copy-if-different for each file installed.  This will not change the 
time it takes to install the first time very much.  Later times may be a 
bit slower but not compared to the savings from not rebuilding projects.

Please add this as a feature request here:

http://www.cmake.org/Bug

-Brad