bug#74783: gcc@4.9.4 fails to build

bug#74783: gcc@4.9.4 fails to build

[gay--- via Bug reports for GNU Guix]

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Hi, trying to run "guix shell hugs" results in error as its dependency 
fails to build.

Attached gzip compressed log.

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bug#74783:

[Gábor Stefanik]

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Currently bisecting this issue, the range I have right now is
3db653a2c235207b347849200eeb3f89bb4ba51 ..
9dbaa10aa755f33f8050082bb443d1f9eabf6e21

That's a 3 minute commit time difference, but containing 216 commits.
Nothing immediately obvious to my eyes. Many (most?) of the intermediate
commits die well before they would start building gcc-4.9.4, making
bisection incredibly tedious.

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bug#74783:

[Gábor Stefanik]

Between 2.35 and 2.38, glibc added the following code to sys/mount.h:

> #ifdef __has_include
> # if __has_include ("linux/mount.h")
> #  include "linux/mount.h"
> # endif
> #endif

This is then used to conditionally define structures that would be in
linux/mount.h, in case linux/mount.h is either unavailable or missing
those structures.

The __has_include directive was added to GCC in v5.0, and didn't
become standard until C++17. Unfortunately, this causes the first
#ifdef to fail, and so linux/mount.h is never checked. Any code trying
to include both sys/mount.h and linux/mount.h will therefore fail when
compiled with GCC < 5.0 and glibc >= 2.38.

Possible solutions:
* Patch sys/mount.h to make it include linux/mount.h unconditionally
on all platforms using the Linux kernel.
* Somehow remove linux/mount.h from GCC 4.9's
sanitizer_platform_limits_posix.cc. Unfortunately it's an indirect
include from linux/fs.h.