[doc] remove -dither arg to imagemagick, in building PDFs
* doc/Makefile.am: arguments for the 'convert' programme seem to have
changed incompatibly at some stage - just remove the problematic -dither.
diff --git a/doc/Makefile.am b/doc/Makefile.am
index 4e0ca66..c3016b2 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -15,7 +15,7 @@
SUFFIXES = .png .eps .dia .pdf
DIATOPNG = dia -t png -e
DIATOEPS = dia -t eps -e
-PNGTOEPS = convert -dither -antialias -contrast -despeckle
+PNGTOEPS = convert -antialias -contrast -despeckle
PNGTOPDF = $(PNGTOEPS)
EPSTOPDF = epstopdf