Default to disabling ImageMagick (Bug#33587)
ImageMagick has continuing stability and security problems, suggesting that 'configure' should disable it by default. See Glenn Morris's notes at: https://lists.gnu.org/r/emacs-devel/2018-12/msg00036.html * INSTALL, etc/NEWS, nt/INSTALL.W64: Mention this. * configure.ac (imagemagick): Default to off.