bug#30756: GCC >= 6 '-isystem' and C_INCLUDE_PATH behavior changed, breaking

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hello,

Ludovic Courtès <ludo@gnu.org> writes:

> Hi,
>
> Reza Housseini <reza.housseini@gmail.com> skribis:
>
>> So what is the workaround for this bug when one wants to use gcc >= 6.0.0
>> with a cmake build system?
>
> Guix has been using GCC 7.x as its default compiler for some time, and
> everything works well, CMake or not.  However, we also switched to using
> ‘CPATH’ instead of ‘C_INCLUDE_PATH’ to work around this bug.
>
> HTH!
>
> Ludo’.

As has been mentioned by Giel van Schijndel earlier in this bug report,
using CPATH causes warnings to be emitted for core libraries such as
glibc since the headers found in CPATH are not (rightfully) treated as
system headers (and thus not muted by GCC).

That has bitten me here, where I was mislead to think there was a build
issue with the latest Inkscape:
https://gitlab.com/inkscape/inkscape/issues/807.  Many packages will end
up having to disable warnings to cope with this behavior.

It'd be nice to find a correct solution, but it seems I can't even make
the build system of Inkscape work after switching from CPATH to
CPLUS_INCLUDE_PATH and stripping it from any glibc/gcc include
directories (I don't get the "stdlib.h: No such file or directory."
error anymore, but I now get:
"/gnu/store/zw5f5g5aqlxam3imaylfla0i98nkridf-glibc-2.30/include/bits/errno.h:26:11:
fatal error: linux/errno.h: No such file or directory" instead, which I
don't understand).

I also tried moving the glibc include directory to the end of
CPLUS_INCLUDE_PATH and it would still wouldn't be happy.  Hmmph!

Maxim