[yocto] Boost build failure for x32 target in thud

[MikeB]

I'm trying to upgrade my project from sumo to thud.

I'm targeting an x86_64 machine with the x32 ABI.

Boost do_compile() fails with several instances like the following.

| /home/mabnhdev/boost-issue-x32-repro/build/tmp/work/x86_64_x32-poky-linux-gnux32/boost/1.68.0-r0/recipe-sysroot/usr/include/bits/long-double.h:60:10:
fatal error: bits/long-double-64.h: No such file or directory
|  #include <bits/long-double-64.h>
|           ^~~~~~~~~~~~~~~~~~~~~~~
| compilation terminated.
| ...failed updating 6 targets...
| WARNING: exit code 1 from a shell command.
| ERROR: Function failed: do_compile (log file is located at
/home/mabnhdev/boost-issue-x32-repro/build/tmp/work/x86_64_x32-poky-linux-gnux32/boost/1.68.0-r0/temp/log.do_compile.15399)
ERROR: Task (/home/mabnhdev/boost-issue-x32-repro/meta/recipes-support/boost/boost_1.68.0.bb:do_compile)
failed with exit code '1'

The failure can be reproduced by trying to build boost for an
'intel-corei7-64' machine with 'x86-64-x32' default tuning.

The attached simple bash script will reproduce the problem.

https://gist.github.com/mabnhdev/42d4775f51df4bbe675ce0a0c9c4c4b0

I'd appreciate some guidance towards fixing this issue.

Regards, Mike