Revise mkversion invocation

[dg0yt]

This is an untested rewrite of the line invoking mkversion on Windows, based on past vcpkg experience. (The current port version 4.4.2 doesn't invoke mkversion.)

The original "\"...\" ...." fails for some configurations.
It is a single CMake list item, with embedded quotes which go into a build system rule which may or may not get the inner quotes correctly for the actual invocation of the command.

The rule of thumb is:

• Quote for CMake.
  There is only outer quotes for items in a cmake list.
• Do not assume a particular shell for commands executed by the build system.
  There are different generators for different build systems.
  With ninja build files, commands may be executed via a shell or directly, depending on the system.
  With any type of inner quotes, portability will be limited.

The new line is a CMake list of command and items, with quoting for CMake when items can expand with special characters inside.
(Now this newest version contains an embedded CMake variable ${MKV_VERSION_OPT} which I didn't quote because I assume it is of list type, i.e. potentially expanding to multiple command line arguments.)

[MarkCallow]

What configurations does this fail on?

MKV_VERSION_OPT is a list. It is set at line 128.

[dg0yt]

This change was originally written for the Ninja generator, Windows, with MSYS2 bash in PATH - this is what is used in the vcpkg port.

The differences, https://ninja-build.org/manual.html#ref_rule_command:

On Unixes ... The Ninja command variable is passed directly to sh -c, which is then responsible for interpreting that string into an argv array.

On Windows ... Ninja passes the command string directly to CreateProcess.

I cannot quickly check the actual contents, or the output for VS generators. My MSVC environment is unprivileged access to vcpkg CI.