Cleaning up autoconf

I made the indentation of the configure.ac files more consistent.  My changes
are mostly cosmetic.  Configure options now use AS_HELP_STRING so `./configure
--help` is more consistent.  There were also some if-else blocks like:

	if test $with_imagemagick = "no" ; then {
		AM_CONDITIONAL(WITH_IMAGEMAGICK,false)
	} else {
		AM_CONDITIONAL(WITH_IMAGEMAGICK,true)
	} ; fi

that I shortened into the equivalent:

	AM_CONDITIONAL(WITH_IMAGEMAGICK, test "x$with_imagemagick" = "xyes")

I also erased some commented-out lines in configure.ac, with the reason that if
they are needed again they can be easily rewritten. As far as I know, these
changes have no effect on compilation.