Expose whisper.cpp version as a static string

Add a `WHISPER_CPP_VERSION` static that contains the version number of whisper.cpp that whisper-rs was built with. This can be useful for downstream applications for debugging and logging purposes.

This is implemented using [build script metadata](https://doc.rust-lang.org/cargo/reference/build-scripts.html#the-links-manifest-key) for whisper-rs-sys to expose the version number of whisper.cpp defined in `CMakeLists.txt` as metadata to crates that depend on it. Then whisper-rs accesses that build metadata and defines its own internal compile-time environment variable containing the same value, which is then assigned to a static variable.

It seemed better to expose this information through build metadata rather than defining a static in whisper-rs-sys directly, despite the extra indirection, so as to keep whisper-rs-sys as pure bindings.

build.rs

@@ -0,0 +1,5 @@
+use std::env;
+
+fn main() {
+    println!("cargo:rustc-env=WHISPER_CPP_VERSION={}", env::var("DEP_WHISPER_WHISPER_CPP_VERSION").unwrap());
+}