build: add support for building CUDA files with clang

This avoids using the CUDA SDK at all; instead, we provide a minimal
reimplementation of the basic functionality that lavfi actually uses.
It generates very similar code to what NVCC produces.

The header contains no implementation code derived from the SDK.
The function and type declarations are derived from the SDK only to the
extent required to build a compatible implementation. This is generally
accepted to qualify as fair use.

Because this option does not require the proprietary SDK, it does not require
the "--enable-nonfree" flag in configure.

Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>