Build: Reduce autogen.sh to trivial stub

Build: Reduce autogen.sh to trivial stub

Use autoreconf instead of invoking the various Auto tools
separately. Get rid of the Darwin-specific guesswork -- it does
not make sense to handle this at the level of libsigrokdecode.

People should set up their ACLOCAL_PATH themselves as appropriate
for their own system; just as they already need to set up various
other paths.