ci: Run autogen.sh without `--maintainer` in ci_verify_configure.sh

The autogen.sh script is not user-serviceable in the 'libpng16' branch,
which implies the following:

- It requires the `--maintainer` option.
- It should not be run by the CI tooling.

Starting from the branch 'libpng18' onwards, the autogen.sh script
becomes not only user-serviceable, but also mandatory, which implies:

- It should be run by the CI tooling.
- It does not require the `--maintainer` option.

Removing the option `--maintainer` from ci_verify_configure.sh should
not only simplify the verification script, but also catch errors that
might occur in the above-mentioned scenarios.

This is a cherry-pick of commit 2cd45a9728fa054ccf17e4cac5a9dd77f46251a0
from branch 'libpng18'.
1 file changed