Some versions of Clang do not properly pass include paths to the
assembler when LTO is enabled. That's a problem for the DEMO_EMBED macro
used by the demos, since it relies on .incbin. To make matters even
better, compilation fails, but doesn't return an error code; the
resulting binary is simply broken. Fortunately we don't really care
about LTO for the demos, so we can simply disable it. See also
https://github.com/llvm/llvm-project/issues/112920
Thanks to Giovanni for tracking down some of the pieces of this issue.