Make nvenc header external

configure

@@ -261,6 +261,7 @@ External library support:
   --enable-libzvbi         enable teletext support via libzvbi [no]
   --disable-lzma           disable lzma [autodetect]
   --enable-decklink        enable Blackmagick DeckLink I/O support [no]
+  --enable-nvenc           enable NVIDIA NVENC support [no]
   --enable-openal          enable OpenAL 1.1 capture support [no]
   --enable-opencl          enable OpenCL code
   --enable-opengl          enable OpenGL rendering [no]
@@ -1393,6 +1394,7 @@ EXTERNAL_LIBRARY_LIST="
     libzmq
     libzvbi
     lzma
+    nvenc
     openal
     opencl
     opengl
@@ -2389,6 +2391,7 @@ libxvid_encoder_deps="libxvid"
 libutvideo_decoder_deps="libutvideo"
 libutvideo_encoder_deps="libutvideo"
 libzvbi_teletext_decoder_deps="libzvbi"
+nvenc_encoder_deps="nvenc"
 
 # demuxers / muxers
 ac3_demuxer_select="ac3_parser"
@@ -4342,6 +4345,7 @@ die_license_disabled gpl x11grab
 
 die_license_disabled nonfree libaacplus
 die_license_disabled nonfree libfaac
+die_license_disabled nonfree nvenc
 enabled gpl && die_license_disabled_gpl nonfree libfdk_aac
 enabled gpl && die_license_disabled_gpl nonfree openssl
 
@@ -4916,6 +4920,7 @@ enabled libxavs           && require libxavs xavs.h xavs_encoder_encode -lxavs
 enabled libxvid           && require libxvid xvid.h xvid_global -lxvidcore
 enabled libzmq            && require_pkg_config libzmq zmq.h zmq_ctx_new
 enabled libzvbi           && require libzvbi libzvbi.h vbi_decoder_new -lzvbi
+enabled nvenc             && { check_header nvEncodeAPI.h || die "ERROR: nvEncodeAPI.h not found."; }
 enabled openal            && { { for al_libs in "${OPENAL_LIBS}" "-lopenal" "-lOpenAL32"; do
                                check_lib 'AL/al.h' alGetError "${al_libs}" && break; done } ||
                                die "ERROR: openal not found"; } &&