fix: Move gnulib/lib/error.h to gnulib-error.h

[blorente]

This is just an early attempt, you can probably think of a better way to fix it.

The problem I'm trying to solve: For good reasons, our gcc toolchain includes libc6 with -isystem on compilation. libc6 has a header, called error.h, which is guarded with the guard _ERROR_H.
M4's gnulib also has a header called error.h with the same guard. Because libc6 comes before gnulib in the -isystem inclusion order, we don't get the symbols from gnulib's error.h. Fun.

The solution we're using in the meantime is to add -I <path/to/gnulib> to the extra copts, because -I resolves as "last one wins", so gnulib gets picked up first: blorente@6238498#diff-64fffd7f2fee0a5a182cc297f3c260f7820729dd4f2855c52aeac25fa5d65271R37-R46

We'd like to avoid maintaining this patch, and instead rely on extra_copts. However, because we shouldn't depend on canonical names, we need to inject the repository name.

As I said, this is just an early attempt, happy to modify it to whatever suits you.

[jmillikin]

I'm not opposed to the feature as stated, but for the specific case of gnulib shadowing the global system <error.h> it might just be easier to rename that file to gnulib-error.h within the m4 build and patch the #include "error.h" lines accordingly. It can go onto the pile of other horrible gnulib build patches.

That approach would also be easier to maintain from a cross-platform perspective, since it would work the same regardless of compiler or system libc.

Note that error.h is being included with #include "error.h" and not #include <error.h>, and according to the GCC documentation I would expect the gnulib version of that header to be picked up before the libc6 version (since -iquote dirs are searched before -isystem). It's possible there are other headers that are also getting shadowed.

[blorente]

Note that error.h is being included with #include "error.h" and not #include <error.h>, and according to the GCC documentation I would expect the gnulib version of that header to be picked up before the libc6 version (since -iquote dirs are searched before -isystem).

I don't see the same thing, In my compilation gnulib is included as an -isystem:

 -isystem external/rules_m4++m4_repository_ext+m4_v1.4.18__cfg02A699F7/gnulib/config-linux -isystem bazel-out/k8-opt-exec-ST-d57f47055a04/bin/external/rules_m4++m4_repository_ext+m4_v1.4.18__cfg02A699F7/gnulib/config-linux -isystem bazel-out/k8-opt-exec-ST-d57f47055a04/bin/external/rules_m4++m4_repository_ext+m4_v1.4.18__cfg02A699F7/gnulib/_virtual_includes/gnulib 

Whatever the reason, I think the solution you presented is worth a shot. The risk there is that the headers from libc6 introduce some symbol that they shouldn't, but it seems like a small risk.

[blorente]

@jmillikin I've made the changes and confirmed that it works with our use case. I'll fix the CI failures in a moment, but please let me know what you think.
I've also added a .bazelversion file, because the repo didn't work on Bazel 9 for me.