Hi folks,

I was bitten by this, and it seemed like a few people online had similar
issues (for example https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65794).

We run a configure script from another configure script, to generate auto-build.h.
Secondary configure might fail.
This failure isn't fatal, and we continue running a configure script.
The output is obscured by a lot of configure messages.
We fail later auto-build.h not existing.

I added a test to see if making auto-build.h succeeded and fail & warn
otherwise. As an added bonus, it means the secondary configure
config.log stays around to inspect.

Let me know what you think.