Summary:
When building the rts with ghc (e.g. using ghc as a c compiler), ghc's
"Value Add"[1] is, it includes adding `-include /path/to/ghcversion.h`.  For
this it looksup the rts package in the package database, which--if
empty--fails.  Thus to allow compiling C files with GHC, we add the
`-ghc-version` flag, which takes the path to the `ghcversion.h` file.

A `-no-ghc-version` flag was omitted, as at that point it becomes
questionable why one would use ghc to compile c if one doesn't
any of the added value.

--

[1] from `compiler/main/DriverPipeline.hs`
>        -- add package include paths even if we're just compiling .c
>        -- files; this is the Value Add(TM) that using ghc instead of
>        -- gcc gives you :)

Reviewers: bgamari, geekosaur, austin

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D4135