Fixed building with OpenCL, but without the ocl module.

HAVE_opencv_ocl implies HAVE_OPENCL, so checking for both is not
necessary.
Fixed CMake warnings/bugs caused by #1670 and #1714

include() doesn't create a variable scope, so the settings of
CMAKE_MODULE_PATH and CMAKE_FIND_ROOT_PATH_MODE_* were polluting
everything included after OpenCVDetectCUDA.cmake.

Also, FindCUDA includes FindPackageHandleStandardArgs, which includes
CMakeParseArguments, which causes warnings related to policy CMP0017.
Setting it to NEW seems safe enough.
Merge pull request #1718 from SpecLad:have-opencv-ocl
Merge pull request #1720 from SpecLad:cmp0017