os/mac/pkgconfig/15: add bzip2.pc for rust formulae

[cho-m]

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Mainly since the only way to avoid bundled bzip2 in rust formulae is via pkg-config - https://github.com/alexcrichton/bzip2-rs/blob/master/bzip2-sys/build.rs#L16-L23

Similar to pkgconfig file created inside bzip2 formula based on https://gitlab.com/bzip2/bzip2/-/blob/master/bzip2.pc.in (note that we currently only ship shared libraries & pkg-config file on Linux, so using brew bzip2 is particularly bad option on macOS).

Removed bindir= and -I${includedir} in Cflags.

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Only added macOS 15 for initial discussion. If acceptable, can add older macOS versions.

[Bo98]

Didn't even realise there was headers in the SDK for this. Nice!

Removed bindir=

Any reason for this? Seems safe enough to have ${exec_prefix}/bin.

If acceptable, can add older macOS versions.

Seems like it's good for all OS versions (though note that pre-10.14 files are slightly different).

macOS 12 and later use v1.0.8.
macOS 11 and earlier use v1.0.6.

Also add a test to test/os/mac/pkgconfig_spec.rb.

[carlocab]

note that we currently only ship shared libraries & pkg-config file on Linux, so using brew bzip2 is particularly bad option on macOS

We should probably fix that.

[cho-m]

Removed bindir=

Any reason for this? Seems safe enough to have ${exec_prefix}/bin.

No major reason so can add.

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note that we currently only ship shared libraries & pkg-config file on Linux, so using brew bzip2 is particularly bad option on macOS

We should probably fix that.

Part of issue is latest bzip2 stable release only cleanly supports Linux .so and would require manual effort to get .dylib. Pkg-config is also not officially supported yet, but worth having as projects expect it due to major Linux distros.

Future 1.1.0 fixes this, but slightly complicates things due to switch to meson/cmake so need to be careful about dependency loops. Should be fine as long as they continue cmake support as we use bundled bzip2 on Linux.