Port to Windows/Visual Studio

[denniskb]

@oikonom This is fairly simple. We don't care about finding equivalent options/commands for VS so it's just a question of guarding VS from seeing linux-specific stuff.

[denniskb]

To elaborate: Spice uses Linux- and GCC-specific commands/options to various degrees.

• The source code and CMakeLists.txt use very few.
• The build scripts and vscode tasks make heavy use of platform-specific code.

This task is not about porting the entirety of Spice to Windows, but just to get it to build so Windows-users can at least run the samples. For this purpose:

• All build scripts and vscode tasks can be completely ignored. We do not strive to support those on Windows. The user will have to build everything manually.
• We do not need to find equivalent options for the Linux/GCC-specific ones inside CMakeLists.txt It is sufficient to hide these options from MSVC (and rely on CMake's default options) just to ensure successful compilation. One exception to this is our kahan_sum implementation which won't work properly unless float optimizations are suspended just for its operator+=. That means we have to find the MSVC equivalent of GCC's __attribute__((optimize("-fno-fast-math"))).
• Confirm that everything works by running the unit tests.

[denniskb]

TODO: Add disclaimer to readme that full performance on Windows cannot be achieved due to lack of async support (only available with Windows Datacenter Edition).