Please support linking against system bzip2 without pkg-config

[mgorny]

The following logic:

https://github.com/alexcrichton/bzip2-rs/blob/3032f3790742bffda521e54d14429f459e078eba/bzip2-sys/build.rs#L15-L24

makes it possible to use system bzip2 library only if it does supply a pkg-config file. However, as of today bzip2 does not install a pkg-config file. As such, this is either dead code or entirely reliant on custom third-party patching.

Could you please add a fallback logic that could be used to link against vanilla bzip2 install? For example, blosc2-sys does that via supporting BLOSC2_INSTALL_PREFIX envvar:

https://github.com/milesgranger/blosc2-rs/blob/7963c3c1c0cc4276d158df5d8bde87430985737c/blosc2-sys/build.rs#L72-L79