QtWebEngine doesn't allow cross build if you don't have a working host 32 bits compiler

When cross-building Qt with WebEngine for a raspberry pi 3 (ARM 32 bits) on my debian sid amd64, I got a compilation failure :

/usr/include/stdint.h:26:10: fatal error: bits/libc-header-start.h: No such file or directory

This failure is easily fixed by installing the 32 bits development files for my host compiler, but the configure script issued no warning about this, and it was quite tricky to find out the real problem.

Buildroot has the same issue and is preparing a patch to have a warning on their own.