add $(WithHappyHc) variable, to be set in build.mk (defaulting to
ghc-0.29, same as $(WithGhcHc)).
#
#################################################################################
+# The compiler you'd like to use to compile Happy
+WithHappyHc = ghc-0.29
+
# HappyHcOpts gives the flags to pass to the Haskell compiler used
# to compile the Happy source coed
HappyHcOpts =