Fix path so gphelp will work.

Index: doc/gphelp.in
--- doc/gphelp.in.orig
+++ doc/gphelp.in
@@ -136,7 +136,7 @@ sub get_docdir {
       $d =~ s,doc/\.\./doc,doc,;
     }
   }
-  return $d;
+  return "${PREFIX}/share/doc/pari";
 }
 
 sub init {
