Improved CMake build

[magestik]

• I removed the FindCUDA.cmake because it was not working correctly contrary to the one shipped with the CMake (latest version).
• The squish/psh/bc6h/bc7 libraries were required to link any program with nvtt so I embedded them in the nvtt/nvimage libraries (using CMake OBJECT libraries). I think the VS solutions will need an upgrade too, but Visual allows libs to be merged easily. (I also updated the minimum CMake version required to use this feature)
• I added some CMake command to export targets and make it easier to link with nvtt from an external (CMake-based) project.

Sorry about the indentation ...

[magestik]

Any news ? (did you receive my emails about a month ago ?)