Add Stack files for different GHC versions

stack-ghc8.0.2.yaml

@@ -0,0 +1,11 @@
+resolver: lts-9.21 # ghc 8.0.2
+
+packages:
+- . # GF, PGF
+- src/runtime/haskell-bind # PGF2 (requires C runtime to be installed)
+
+extra-deps:
+
+flags:
+  gf:
+    server: true