Treat DirectSound just like any other standard Windows SDK library.

Treat DirectSound just like any other standard Windows SDK library.

Currently the CMake build system goes through complicated logic to
find the DirectX SDK. This is completely unnecessary, because modern
versions of the Windows SDK include dsound.h and dsound.lib in the
standard paths, so it can be treated just like the other Windows OS
libraries and we can safely assume it is available.

This commit simplifies the logic and will allow the vcpkg portaudio
port to align with portaudio stream, by getting rid of this patch:
  https://github.com/Microsoft/vcpkg/blob/95f9ce56f32618cceda97d54af16e3d6c57fc4b3/ports/portaudio/find_dsound.patch

I have verified that PortAudio CMake still builds with this commit
with the following toolchains:
 - Windows Visual Studio 2017 native CMake support
 - mingw-w64 on Windows (msys2)
 - mingw-w64 cross-compiling from Debian Unstable