imagemagick: bump version, fix build with zlib

The recent zlib bump broke imagemagick. This has been fixed upstream
in 6.7.5, but the xml2-config fix is still not upstream and 6.7.5
needs autoconf 2.67 to autoreconf (and we have 2.65), so we cannot
easily use that.

Instead move to the most recent version using autoconf 2.64 and
backport the fix from imagemagick svn. At the same time also
ensure zlib+bzip2 support is picked up if enabled.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>