build: kill maintainer mode, always rebuild by default

build: kill maintainer mode, always rebuild by default

The automake manual recommends against the use of disabling
maintainer mode by default:

https://www.gnu.org/software/automake/manual/automake.html#maintainer_002dmode

because when it is disabled, the user gets no indication if they
touch a file that would normally require a rebuild.  Automake
1.11 changed things so that AM_MAINTAINER_MODE([enable]) will set
the mode to enabled by default; but RHEL 5 still uses automake 1.9,
where AM_MAINTAINER_MODE did not recognize an argument, and
therefore disables maintainer mode by default.  Having the default
be different according to which version of automake built the
project is annoying, and I _have_ been bitten on RHEL 5 rebuilds
where the default disabled mode led to silently incorrect builds.

The automake manual admits that being able to disable maintainer
mode still makes sense for projects that still store generated
files from the autotools in version control; but we have dropped
that for several years now.  As such, it's finally time to just
ditch the whole idea of maintainer mode, and unconditionally
rebuild autotools files if a dependency changes, without offering
a configure option to disable that mode.

* configure.ac (AM_MAINTAINER_MODE): Drop.

Signed-off-by: Eric Blake <eblake@redhat.com>