Architecture-specific C headers on Debian-based OSs now accounted

Rencely the building of GCC was allowed on Debian-based systems that have
their basic C library in architecture-specific directories, like
`/usr/lib/x86_64-linux-gnu'. However, these systems also have their headers
in non-standard locations, for example `/usr/include/x86_64-linux-gnu' and
this caused a crash on a new Ubuntu system.

/usr/include/stdio.h:27:10: fatal error: bits/libc-header-start.h: No
such file or directory
   27 | #include <bits/libc-header-start.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.

The reason it didn't cause problems on other Ubuntus that we tested before
was historic: In the old days, we would ask Ubuntu systems to install
multilib features to have GCC. Because they had installed those features,
this problem didn't show up! But this wasn't mandatory!

With this commit, the `CPATH' environment variable is set (similar to how
`LIBRARY_PATH' was set) and this fixed the problem on a clean Debian
virtual machine.

This bug was reported by Sebastian Luna Valero.

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