Tweak build.mk.sample

Now that we can build dyn-only, we don't need to force
DYNAMIC_BY_DEFAULT to NO. We just set GhcLibWays appropriately.

mk/build.mk.sample

@@ -30,8 +30,7 @@
 # A development build, working on the stage 2 compiler:
 #BuildFlavour = devel2
 
-GhcLibWays = v
-DYNAMIC_BY_DEFAULT = NO
+GhcLibWays = $(if $(filter $(DYNAMIC_BY_DEFAULT),YES),dyn,v)
 
 # Uncomment this to get prettier build output.
 # Please use V = 1 when reporting GHC bugs.