imagemagick: explicitly specify ghostscript font directory

imagemagick: explicitly specify ghostscript font directory

It appears that Imagemagick needs to be configured with
 --with-gs-font-dir=/usr/share/fonts/gs else it will not find
the ghostscript fonts and /etc/ImageMagick-6/type-ghostscript.xml
will contain the wrong font paths. You end up with messages like:
Magick: unable to read font `(null)' @ error/annotate.c/RenderFreetype/1153
They are non-fatal but could get annoying fast if you are using 'convert'
on a lot of files.

Without patch we end up with (depending on what is available on the host):

  Font Configuration:
  Apple fonts       --with-apple-font-dir=default
  Dejavu fonts      --with-dejavu-font-dir=default      none
  Ghostscript fonts --with-gs-font-dir=default  /usr/share/fonts/type1/gsfonts/
  Windows fonts     --with-windows-font-dir=default     none

And with the patch:

  Font Configuration:
  Apple fonts       --with-apple-font-dir=default
  Dejavu fonts      --with-dejavu-font-dir=default      none
  Ghostscript fonts --with-gs-font-dir=/usr/share/fonts/gs      /usr/share/fonts/gs/
  Windows fonts     --with-windows-font-dir=default     none

[Peter: reworked/extended commit message]
Signed-off-by: Robert Sohn <grepper@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>