Clang requires compiler targets in order to work

[reynarzz]

I encountered some minor issues trying to build using Clang on Windows, it looks like we need to turn on a few compiler features.
I added this to the CMakeLists.txt and it's compiling.

if(WIN32)
target_compile_options(${PROJECT_NAME} PRIVATE 
"-mavx"
"-march=native"
"-mssse3"
"-msse4.1"
"-mpclmul"
"-maes"
)
endif()

However, some guards must be added to check if the processor contains these features.
On another note, Clang for Android (NDK) does not need this

[robinlinden]

Thank you! I've opened a PR adding support for building with clang-cl as well as adding CI to make sure it remains working. :)

Looks like the #pragma ... target(...) macros in libsodium weren't ifdef'd in when building with clang-cl even though they're required, so manual feature-detection it is. This might be resolved in a newer version, so hopefully we can drop this once we've updated to target a more recent release of libsodium.