Spurious linkage errors to GCC on Linux about brew

[carlocab]

@p-linnane also mentioned something about an audit failure, but I don't see it in the CI log at Homebrew/homebrew-core#171701, so I'll let Patrick chime in on how to reproduce that.

[MikeMcQuaid]

Thanks @carlocab for opening!

Either brew linkage shouldn't complain about this or it should be fixed.

[p-linnane]

Repro: check out the PR, build and then:

==> brew linkage --cached sherlock
System libraries:
  /lib/x86_64-linux-gnu/libc.so.6
  /lib/x86_64-linux-gnu/libm.so.6
Homebrew libraries:
  /home/linuxbrew/.linuxbrew/opt/gcc/lib/gcc/current/libgcc_s.so.1 (gcc)
  /home/linuxbrew/.linuxbrew/opt/gcc/lib/gcc/current/libstdc++.so.6 (gcc)
Indirect dependencies with linkage:
  gcc
Dependencies with no linkage:
  certifi

[carlocab]

I was referring to this audit failure, which I can't find in the log:

Full audit --formula sherlock --online --new output
  sherlock
    * Formulae in homebrew/core should not have a Linux-only dependency on GCC.
  Error: 1 problem in 1 formula detected.

[Bo98]

That audit is correct. "Fixing" the linkage error by adding a dependency would fail that audit. The bug is that we shouldn't be flagging indirect (or undeclared) GCC linkage in brew linkage because of how the rpaths are set up. Every formula depends on GCC (or rather near enough most that we call it every formula) and we have magic in brew that auto-installs GCC when the system GCC is too old.

[carlocab]

Right, ok, so the audit failure appears if you add a Linux-only dependency on GCC, which explains why I couldn't find it in the log.

[carlocab]

Should be fixed by #17306.