ffmpeg ERROR: libnpp not found in windows

I managed to successfuly cross compile ffmpeg under linux targeting Windows 64 bit with --enable-libnpp included.

My environment is Ubuntu Server 16.10 64bit.
After a fresh installation I installed MinGW using the command:

sudo apt-get install mingw-w64

First I successfully compiled the Linux version with the --enable-libnpp option activated following the instructions on the NVIDIA dev site Compile Ffmpeg with NVIDIA Video Codec SDK.
In order to do that you need to install the CUDA Toolkit. Just follow the instructions and the package installer will create the symbolic links (I have the CUDA Toolkit 8.0):

/usr/local/cuda/include/ -> /usr/local/cuda-8.0/targets/x86_64-linux/include
/usr/local/cuda/lib64/ -> /usr/local/cuda-8.0/targets/x86_64-linux/lib

This should provide Configure the right path to find the correct libraries and headers.
The command line I have used to compile the linux version of ffmpeg is:

./configure --enable-nonfree --disable-shared --enable-nvenc --enable-cuda --enable-cuvid --enable-libnpp --extra-cflags=-I/usr/local/cuda/include/ --extra-ldflags=-L/usr/local/cuda/lib64/

The problem you got is that when using cross-compilation you need to provide Configure the right path where to find headers and library for the Windows version of the libnpp library.
From the CUDA Toolkit Download page mentioned above I simply downloaded the exe(local) version of the Windows package.
Under the root of my working folder I created a folder called tmp where I copied the subfolders I found under npp_dev inside the package cuda_8.0.61_win10.exe:

cuda_8.0.61_win10.exe\npp_dev\lib -> tmp/lib  cuda_8.0.61_win10.exe\npp_dev\include -> tmp/include

As final step I launched Configure once again using the following parameters:

./configure --arch=x86_64 --target-os=mingw32 --cross-prefix=x86_64-w64-mingw32- --pkg-config=pkg-config --enable-nonfree --disable-shared --enable-nvenc --enable-cuda --enable-cuvid --enable-libnpp --extra-cflags=-I/usr/local/include --extra-cflags=-I/usr/local/cuda/include/ --extra-ldflags=-L/usr/local/cuda/lib64/ --extra-cflags=-I../tmp/include/ --extra-ldflags=-L../tmp/lib/x64/

The compilation completed successully. When I copied the ffmpeg.exe file to Windows and tried to execute it I got an errore message saying the executable was missing some npp_*.dll.
From the package cuda_8.0.61_win10.exe I copied all the dlls included into the folder npp\bin to the same directory I put ffmpeg.exe.
After that the application run normally and a simple conversion from a 4K file completed as expected.

Actually I went nuts about ffmpeg is not building with the same problem. I fianally managed to get it worked under Windows 10 x64:

1. Download msys2 from https://www.msys2.org/ and install all packages with Pacman

2. pacman -Su

3. pacman -S make

4. pacman -S diffutils

5. pacman -S yasm

6. pacman -S mingw-w64-x86_64-gcc

7. pacman -S mingw-w64-x86_64-toolchain

8. add pkgconfig to environment variable PKG_CONFIG_PATH=$PKG_CONFIG_PATH:/usr/local/lib/pkgconfig

9. Add additional installed toolchain to path: PATH=$PATH:/opt/bin

10. Start mingw64 version: C:\msys64\msys2_shell.cmd -mingw64

11. Download and install Cuda from nVidia https://developer.nvidia.com/cuda-downloads?target_os=Windows&target_arch=x86_64&target_version=10&target_type=exenetwork

12. Extract the downloaded file e.g. cuda_11.2.2_461.33_win10.exe with 7zip locally

13. Copy cuda_nvcc\nvcc\include to your msys2 e.g. C:\msys64\tmp\nvidia_include

14. Copy libnpp\npp_dev\lib\x64 to your C:\msys64\tmp\nvidia_lib\x64

15. Copy libnpp\npp_dev\include to C:\msys64\tmp\nvidia_npp_include

16. git clone https://github.com/FFmpeg/FFmpeg.git to C:\msys64\home\<user>

17. git clone https://github.com/libav/libav to C:\msys64\home\<user>

18. Maybe optional step: git clone https://git.videolan.org/git/ffmpeg/nv-codec-headers.git to C:\msys64\home\<user>

19. make

20. make install

21. Optional because make install should have done this for you: Copy ffnvcodec.pc to C:\msys64\usr\local\lib\pkgconfig

22. Build libav avconv.exe and avprobe.exe are needed for ffmpeg later:

23. cd C:\msys64\home\<user>\libav

24. ./configure

25. make

26. make install

27. Finally build ffmpeg:

28. cd C:\msys64\home\<user>\ffmpeg

29. ./configure --enable-nonfree --disable-shared --enable-nvenc --enable-cuda --enable-cuvid --enable-libnpp --extra-cflags=-I/tmp/nvidia_npp_include --extra-cflags=-I/tmp/nvidia_include --extra-ldflags=-L/tmp/nvidia_lib/x64

30. make

31. make install

32. Copy avconv.exe and avprobe.exe to ffmpeg directory

Done.

Bugfixing:

• Missing DLLs: find x64 missing DLLs on your harddisk or in internet.
• Use dependency walker for analyzing errors
• Download the newest nVidia drivers and use nSight making sure CUVID is supported for your graphic card.

This would seem to be caused by a broken configuration script in the FFmpeg code base. There is no library called npp in recent CUDA distributions, instead on Windows platforms you will have

nppc.libnppi.libnpps.lib

and on linux

libnppc.solibnppi.solibnpps.so

You will either need to modify the configuration system yourself or file a bug request with the project developers to do it for you.

There might still be additional problems building the project with MinGW, but that is way beyond the scope of a Stack Overflow question.