Disable the maintainer mode in the Autotools CI jobs

Currently, the Autotools CI jobs are configured with --enable-maintainer-mode=yes for revisions that do not contain the configure script. Otherwise (e.g. for release branches), the jobs configure the project with --enable-maintainer-mode=no to avoid regenerating the tracked files and test them as-is. Normally, the automatically generated files must not be added to the repo. We have one exception though: src/mtime_iso8601.c is generated with ragel based on src/mtime_iso8601.rl. Since ragel is not universally available, the decision was made to add the automatically generated C source file to the repo. The makefile rule to regenerate the file is activated only in the maintainer mode. The problem is that git does not (and, yes, should not) guarantee any particular order of the file modification timestamps in a newly cloned repository. As a result, the build system in a CI job might or might not, absolutely randomly, decide to update src/mtime_iso8601.c. To eliminate the randomness, to save the time of providing the CI jobs with ragel (given the Autotools build system will be removed soon), it is better if we simply make CI jobs always configure with --disable-maintainer-mode.