Added AM_MAINTAINER_MODE to disable Makefile rebuild rules

Some people might want to check-in unpacked result on make dist into
git. But because git doesn't preserve timestamps it would cause those
automatic "auto-retool" rules to trigger. Sometimes even causing build
breakage if system's autotools version don't match autotools version
used for make dist.

Easiest way around this problem is to simply disable those unnecessary
"maintainer" rebuild rules. Especially given that source is always
freely available via git and therefore there should be no reason to
regenerate any of autotools products in 'make dist'-produced sources.