* autogen.sh: Replace script with a minimalistic wrapper around
mm-common-prepare, autoreconf and configure.
* Makefile.am (ACLOCAL_AMFLAGS): Pick up ${ACLOCAL_FLAGS} from the
environment, so that autoreconf can do its magic.
* configure.ac: Modernize.  Cut loads of old cruft.  Make use of
magic mm-common macros to simplify things.  Declare build/ as the
destination for auxiliary build files.  Add TODO comments to flag
the pieces that still need cutting.  Re-enable autoheader.
* config.h.in: Remove file, and have autoheader generate it.