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` output is more consistent.  Some if-then-else blocks like this:

	if test $with_imagemagick = "no" ; then {
		AM_CONDITIONAL(WITH_IMAGEMAGICK,false)
	} else {
		AM_CONDITIONAL(WITH_IMAGEMAGICK,true)
	} ; fi

are now shorter:

	AM_CONDITIONAL(WITH_IMAGEMAGICK, test "x$with_imagemagick" = "xyes")

I also erased the commented checks in configure.ac, with the reason that if
they are needed they can easily be rewritten. As far as I know, it has no
bad effects on compilation.