[dune_docu.m4] Add conditional CONVERT. This fixes FS#1220.

[[Imported from SVN: r7446]]

m4/dune_docu.m4

@@ -13,6 +13,7 @@ AC_DEFUN([DUNE_DOC_PROGS],[
   AC_CHECK_PROGS([PERL], [perl], [true])
   AC_REQUIRE([DUNE_INKSCAPE])
   AC_CHECK_PROGS([CONVERT], [convert], [true])
+  AM_CONDITIONAL([CONVERT], [test "x$CONVERT" != xtrue])
 ])
 
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