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.
Debug builds are simply too slow to be remotely usable even on a 13700K,
or with an Intel Arc A770 GPU
(both tested to be unusable for anything approaching realtime)
This allows whipser-rs-sys to work when building a cdylib on x86 Linux
in certain conditions.
Without this change, linking can fail with:
```
= note: /usr/bin/ld: /.../target/debug/deps/libwhisper_rs_sys-d9be91f496c91a32.rlib(whisper.cpp.o): warning: relocation against `_ZTVNSt7__cxx1115basic_stringbufIcSt11char_traitsIcESaIcEEE@@GLIBCXX_3.4.21' in read-only section `.text.unlikely'
/usr/bin/ld: /.../target/debug/deps/libwhisper_rs_sys-d9be91f496c91a32.rlib(whisper.cpp.o): relocation R_X86_64_PC32 against symbol `stderr@@GLIBC_2.2.5' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: bad value
collect2: error: ld returned 1 exit status
```
Note that this is one solution. I believe another would be to use the [cmake crate](https://github.com/rust-lang/cmake-rs), which builds with `-fPIC` by default on Linux.
