Fix automake-1.14 am_aux_dir

AC_USE_SYSTEM_EXTENSIONS will cause AC_PROG_CC, which is overridden by
automake-1.14, which will then init (part) of automake, in particular
am_aux_dir expansion, which in turn relies on ac_aux-dir, which is not
initialized at this point, and thus: certain doom (or fun, depending on
your POV and mood :p)

Hence call AC_USE_SYSTEM_EXTENSIONS only after AM_INIT_AUTOMAKE. This,
of course, caused a lot of related macro shuffling.

Tested against automake-1.10 (OSX Lion/XCode version) and automake-1.14
(homebrew version)
AM_SILENT_RULES([yes]) with backwards-compatiblity

Supported since automake-1.11. There is no point in having the very
verbose compile stuff running about, which cannot even silenced properly
with `make -s` by default. Otherwise, `make V=1` or
`--disable-silent-rules` are your friends