Updated opencv.rb to 2.4.5. Added flags to enable gtk+ and openexr.

Updated URL and hash for OpenCV 2.4.5.
OpenCV 2.4.5 requires llvm-gcc/g++ to compile some CUDA
code due to use of a -dumpspecs flag. So I added ENV.llvm.
Added the appropriate flag to turn off CUDA if the --with
argument is missing.

Added options to enable gtk+ and openexr as optional dependencies.
Added the appropriate flags to send to cmake.