Fix OpenSSL detection on Apple silicon

[shawwn]

Currently, when macOS users clone tarsnap and try to run ./configure, M1 and M2 macs will run into an error saying they're missing OpenSSL:

~/ml/tarsnap $ ./configure
checking build system type... aarch64-apple-darwin22.5.0
checking host system type... aarch64-apple-darwin22.5.0
checking target system type... aarch64-apple-darwin22.5.0
...
checking for openssl/rsa.h... no
configure: error: *** OpenSSL header files missing ***

This PR detects whether the system is running arm64, and if so, searches /opt/homebrew instead of /usr/local.

The effect is that all macOS users (whether on Intel or Apple silicon) can clone tarsnap, run autoreconf && ./configure && make -j12 and the build process will work, assuming they've run brew install openssl.

[gperciva]

Hmm. Is the directory change actually related to arm64, or does it depend on the version of homebrew? Also, does MacPorts still exist? The wikipedia page suggests that it's still active.

I definitely agree that there's a problem, but I'm not convinced that this is the right way to fix it. I'm going to continue investigating.

[gperciva]

Ah, found it: homebrew defaults to /opt/homebrew for arm64, and /usr/local for intel. https://docs.brew.sh/Installation

So yes, this PR makes complete sense.

It does raise the question of "why doesn't our check_darwin_paths.m4 support macports, which installs to /opt/local?", but that's a separate issue.

[shawwn]

Ah, found it: homebrew defaults to /opt/homebrew for arm64, and /usr/local for intel. https://docs.brew.sh/Installation

So yes, this PR makes complete sense.

It does raise the question of "why doesn't our check_darwin_paths.m4 support macports, which installs to /opt/local?", but that's a separate issue.

For what it’s worth, I haven’t heard of anyone using macports, and I’ve worked with a lot of macOS devs. Especially now that intel macs are being phased out.

Oh, I should probably mention… before fixing this properly, I was trying to figure out how to pass extra flags to cc and ld so that I could inject my /opt/homebrew OpenSSL include and lib dirs into the build process. I finally got cc with ./configure CCFLAGS=“-I /opt/homebrew…” (I think?) and eventually figured out the corresponding ld flag, but it was obscure.

If you happen to know the proper way to pass -I and -L flags, I’d like to submit a PR explaining that in the build docs. That way if a macports user pops into existence in 2025 after being catapulted through time by a game of Jumanji, they’ll be able to point tarsnap at their /opt/local OpenSSL lib. (Or if someone builds OpenSSL from source and wants to link tarsnap to their local copy for some reason. Though I guess the system OpenSSL might accidentally take precedence in that case.)

[gperciva]

For what it’s worth, I haven’t heard of anyone using macports, and I’ve worked with a lot of macOS devs.

Good to know! That's something that we can't easily pick up from skimming the wikipedia page. :)

As for ./configure flags, at first glance I'd suggest that our instructions for scrypt should just be copied over: https://github.com/Tarsnap/scrypt/blob/master/BUILDING#L42

I suspect that I intended to copy those lines to tarsnap/BUILDING after the next scrypt release had been out for a few months, in case anybody found a problem with it. But that was 7 years ago, so I'm not certain what I was thinking.

[gperciva]

As part of your fixes, could you please rebase this PR on top of current master (in order to get #585, which should allow github actions to run).

[gperciva]

Thanks for the fix! This was merged as part of #594.