deb: build-depend on autoconf >= 2.63

A change to src/configure.in in the unified-build-candidate-3 branch
(commit ed1e9c36ea25bc20a16b97c7a10aaf6a978aadba) introduced the use of
m4 macros which are not present in Hardy's autoconf (2.61).

Jeff Epler determined that autoconf 2.63 introduced the macros:
http://www.mail-archive.com/emc-developers@lists.sourceforge.net/msg11078.html
deb: unconditionally build-depend on imagemagick

Before this commit only the Ubuntu distros (Hardy and newer)
build-dependend on imagemagick, and every other platform tried to build
without it.  But the docs need the 'convert' program from imagemagick
to build, so make that build-dependency unconditional.

Maybe the previous wonky behavior was a left-over from when we used
to support even older distros, that didn't ship with imagemagick?
Or maybe it came about by accident during the lyx-to-asciidoc switch?
I really don't know.