FindCUDA: Fix OSX Clang & no C language enabled

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 in case neither CMAKE_C_COMPILER is defined (project does not use C
language) nor CUDA_HOST_COMPILER is specified manually we should skip -ccbin
and let nvcc use its own default C compiler.
Merge topic 'cuda_osx_clang' into next

d0d2fdd4 FindCUDA: Fix OSX Clang & no C language enabled