groff: Remove indeterminism in manpages

It was caused by including the modification date

pkgs/tools/text/groff/default.nix

@@ -18,7 +18,10 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = false;
 
-  patches = [ ./look-for-ar.patch ];
+  patches = [
+    ./look-for-ar.patch
+    ./mdate-determinism.patch
+  ];
 
   postPatch = stdenv.lib.optionalString (psutils != null) ''
     substituteInPlace src/preproc/html/pre-html.cpp \

pkgs/tools/text/groff/mdate-determinism.patch

@@ -0,0 +1,13 @@
+diff --git a/Makefile.comm b/Makefile.comm
+index 75efc22..b757000 100644
+--- a/Makefile.comm
++++ b/Makefile.comm
+@@ -155,7 +155,7 @@ extraclean: distclean
+ 	     -e "s|@MAN1EXT@|$(man1ext)|g" \
+ 	     -e "s|@MAN5EXT@|$(man5ext)|g" \
+ 	     -e "s|@MAN7EXT@|$(man7ext)|g" \
+-	     -e "s|@MDATE@|`$(SHELL) $(top_srcdir)/mdate.sh $<`|g" \
++	     -e "s|@MDATE@|`date +'%-d %B %Y' -r $(top_srcdir)/ChangeLog`|g" \
+ 	     -e "s|@OLDFONTDIR@|$(oldfontdir)|g" \
+ 	     -e "s|@PDFDOCDIR@|$(pdfdocdir)|g" \
+ 	     -e "s|@SYSTEMMACRODIR@|$(systemtmacdir)|g" \