"The current compiler doesn't support safely installing libraries" error in non multilib case

```
Warning: The current compiler doesn't support safely installing libraries, so
only executables will be available. (Library installation is supported on GHC
8.0+ only)
```

I did bare `cabal v2-install -w ghc-7.6.3 --lib lib1 lib2` non of them uses public multi-libraries.

This warning is bogus.

```
>>> cabal --version
cabal-install version 3.0.1.0
compiled using version 3.0.2.0 of the Cabal library
```

```
>>> ghc-7.6.3 --version
The Glorious Glasgow Haskell Compilation System, version 7.6.3
```