Conditional blocks broken for version checks under ghcjs

[alexfmpe]

Checkout alexfmpe/reflex-project-skeleton@c841533 and run ./reproduce-bug

It seems this conditional isn't being processed properly when under ghcjs.
I managed to build by cloning singleton-bool locally and changing

-if impl(ghc >=7.8)
+if impl(ghc >=7.8) || impl(ghcjs >=7.8) 

I have cabal2nix 2.1.4 on my environment and it seems to handle all cases properly, as long as the compiler version is also passed in. That is,
cabal2nix --compiler ghcjs-8.6 --shell . > shell.nix yields the expected output but
cabal2nix --compiler ghcjs --shell . > shell.nix does not.

[alexfmpe]

Still present in latest reflex-platform develop.
Oddly this only happens when there is an override in the project, even though the cabal version is always 0.1.5