nvenc: drop the hard dependency on CUDA

The code needs only a few definitions from cuda.h, so define them
directly when CUDA is not enabled. CUDA is still required for accepting
HW frames as input.

Based on the code by Timo Rothenpieler <timo@rothenpieler.org>.

configure

@@ -4665,7 +4665,6 @@ if enabled libdc1394; then
 fi
 
 if enabled nvenc; then
-    enabled cuda || check_header cuda.h || die "ERROR: cuda.h not found."
     check_header nvEncodeAPI.h || die "ERROR: nvEncodeAPI.h not found."
     check_cpp_condition nvEncodeAPI.h "NVENCAPI_MAJOR_VERSION >= 6" ||
         die "ERROR: NVENC API version 5 or older is not supported"

libavcodec/nvenc.c

@@ -21,7 +21,6 @@
 
 #include "config.h"
 
-#include <cuda.h>
 #include <nvEncodeAPI.h>
 #include <string.h>
 

libavcodec/nvenc.h

@@ -19,7 +19,6 @@
 #ifndef AVCODEC_NVENC_H
 #define AVCODEC_NVENC_H
 
-#include <cuda.h>
 #include <nvEncodeAPI.h>
 
 #include "config.h"
@@ -29,6 +28,24 @@
 
 #include "avcodec.h"
 
+#if CONFIG_CUDA
+#include <cuda.h>
+#else
+
+#if defined(_WIN32)
+#define CUDAAPI __stdcall
+#else
+#define CUDAAPI
+#endif
+
+typedef enum cudaError_enum {
+    CUDA_SUCCESS = 0
+} CUresult;
+typedef int CUdevice;
+typedef void* CUcontext;
+typedef void* CUdeviceptr;
+#endif
+
 #define MAX_REGISTERED_FRAMES 64
 
 typedef struct NVENCFrame {