[Bug other/84715] New: "./configure --disable-multilib && make" fails due to "g++: error: gengtype-lex.c: No such file or directory"

[krichter at posteo dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84715

            Bug ID: 84715
           Summary: "./configure --disable-multilib && make" fails due to
                    "g++: error: gengtype-lex.c: No such file or
                    directory"
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: krichter at posteo dot de
  Target Milestone: ---

Running `./configure --disable-multilib && make` fails due to

```
g++: error: gengtype-lex.c: No such file or directory
g++: fatal error: no input files
compilation terminated.
make[3]: *** [build/gengtype-lex.o] Error 1
make[3]: *** Waiting for unfinished jobs....
Makefile:2666: recipe for target 'build/gengtype-lex.o' failed
```

on a pretty empty docker image. I recreated the issue on Ubuntu 14.04, 16.04,
17.10 and 18.04 preview as well as Debian oldstable, stable and testing inside
GitLab CI, see https://gitlab.com/krichter/gcc/pipelines/18408388 for details
and
[.gitlab-ci.yml](https://gitlab.com/krichter/gcc/blob/gitlab-ci/.gitlab-ci.yml)
for the build instructions.

In case this is a user error, it'd be nice if `configure` would check that all
conditions for the configured build are fulfilled and otherwise fail with a
helpful error message.

experienced with 4225b5dcdbcadaa3d7605582f735d02b6fc50cdf