[BUG] PCH + Ninja: random parallel build failure

[wavexx]

Describe the bug
Probably not a bug in itself, but I'm logging this here: I just tried to rebuild using ninja & gcc 11. With the default being PCH=On, I sometimes see:

cc1plus: error: /MK404/build/CMakeFiles/MK404.dir/cmake_pch.hxx.gch: created and used with differing settings of '-mrtm' [-Werror=invalid-pch]

This is misleading though. I guess that -mrtm is actually toggled by a different optimization level somewhere. Or maybe it's actually a bug in gcc 11. And since ninja tends to run things more parallel-ly than make, you get this.

ninja -j1 will clear this, as well as disabling pch, as well as just using make ;)

To Reproduce
Use cmake -G Ninja, gcc 11 and ninja -j>5 when building.

Expected behavior
No build failure even when PCH is enabled.

Desktop (please complete the following information):

• OS: debian unstable

Additional context
I might debug this later myself, just logging this for reference.

[vintagepc]

FYI we've seen similar issues running Ninja directly on Mini404 as well; the auto-parallel builds there fail because a certain order of execution/build is required that it doesn't respect.

It'd be nice if we could fix it, sure.