Make stack.yaml an actual symlink to stack-ghc8.6.5.yaml. Add some commented flags in stack files.

stack-ghc7.10.3.yaml

@@ -9,4 +9,9 @@ allow-newer: true
 
 flags:
   transformers-compat:
-    four: true
+    four: true
+#   gf:
+#     c-runtime: true
+#
+# extra-lib-dirs:
+#   - /usr/local/lib