FindCUDA: Fix compilation with Clang on OSX

This is accompanying pull request for: http://www.cmake.org/Bug/view.php?id=14971

When setting default CUDA_HOST_COMPILER we must dereference CMAKE_C_COMPILER,
i.e. /usr/bin/clang should be used instead /usr/bin/cc which is symlink.
Otherwise CUDA thinks it is GCC and issues -dumpspecs which is unknown option
to Clang.

Also we need to ensure CMAKE_C_COMPILER is defined, which can be a case when
project specifies CXX language only.