-W is not a supported make flag on freebsd. Fixes #181.

rebar.config

@@ -27,7 +27,7 @@
 
 {pre_hooks,
  [{compile, "make -C $REBAR_DEPS_DIR/merl all -W test"},
-  {"freebsd", compile, "gmake -C $REBAR_DEPS_DIR/merl all -W test"},
+  {"freebsd", compile, "gmake -C $REBAR_DEPS_DIR/merl all"},
   {eunit,
    "erlc -I include/erlydtl_preparser.hrl -o test"
    " test/erlydtl_extension_testparser.yrl"},