Automatically use CMake for Windows builds

[chklauser]

This PR changes the rdkafka-sys/build.rs script to automatically choose CMake if the target platform is Windows. The cmake-build cargo feature no longer has an effect on Windows. It can still be used to select the CMake build for other targets. The official rdkafka build system remains the default for other targets.

This solves a number of problems:

• Projects using rdkafka/rdkafka-sys no longer have to conditionally enable cmake-build when they reference the crates just to have something that compiles across platforms.
• JetBrains RustRover + rust-analyzer fail to initialize on Windows because the rdkafka-sys/build.rs script fails. In RustRover, even if you otherwise enable the cmake-build feature, there is an early "disovery" execution of the project that uses default features and fails miserably.
• People evaluating the crate have a better experience if they happen to run Windows

I had initially tried to use the cmake-build feature, but there doesn't seem to a reliable mechanism to conditionally add a default feature. One option is to exploit a side effect of the v1 feature resolver: in the rdkafka crate, if we add a dependencies section that is conditional on target_os = "windows" and pass cmake-build as a feature to rdkafka-sys, that feature accidentally remains enabled even if you run cargo test --package rdkafka-sys from the root. This is definitely fixed in the v2 feature resolver, though.