bug#7955: patch for imagemagick header rename

[Chong Yidong]

Glenn Morris <rgm@gnu.org> writes:

> Seems fine on RHEL5. On RHEL4, build still fails.
>
> configure says:
>
> checking IMAGEMAGICK_CFLAGS...
> checking IMAGEMAGICK_LIBS... -lWand -lMagick
> checking for MagickExportImagePixels... no
> checking wand/MagickWand.h usability... no
> checking wand/MagickWand.h presence... no
> checking for wand/MagickWand.h... no
>   Does Emacs use imagemagick?                             yes

ImageMagick-6.0 is almost 7 years old.  Let's just set HAVE_IMAGEMAGICK
back to "no" if AC_CHECK_HEADER doesn't find MagickWand.h, and not try
to handle the magick_wand.h case.