* Disable maintainer mode by default

When maintainer mode is enabled, `make` will not try to run
`aclocal`/`autoconf`/`automake` when corresponding `configure.ac` or
`Makefile.am` is changed. It is very inconvenient for daily development
that these files are changing everyday. Developers are force to re-run
bootstrap and configure when these files are changed.

With maintainer mode disabled, changes of these files will be handled by
`Makefile` automatically, `make` will re-run corresponding commands when
changes of these files are detected.

The purpose of maintainer mode is to prevent updating `Makefile.in` and
`configure` after release. Because CHIP doesn't release these files,
there is no reason to enable maintainer mode.

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