Adds -ghc-version flag to ghc.

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

1 files changed, 8 insertions, 3 deletions

diff --git a/compiler/main/DriverPipeline.hs b/compiler/main/DriverPipeline.hs
index fab7fad9bc..4f7bfbda5c 100644
--- a/compiler/main/DriverPipeline.hs
+++ b/compiler/main/DriverPipeline.hs
@@ -2204,11 +2204,16 @@ touchObjectFile dflags path = do
 -- | Find out path to @ghcversion.h@ file
 getGhcVersionPathName :: DynFlags -> IO FilePath
 getGhcVersionPathName dflags = do
-  dirs <- getPackageIncludePath dflags [toInstalledUnitId rtsUnitId]
+  candidates <- case ghcVersion dflags of
+    Just path -> return [path]
+    Nothing -> (map (</> "ghcversion.h")) <$>
+               (getPackageIncludePath dflags [toInstalledUnitId rtsUnitId])
 
-  found <- filterM doesFileExist (map (</> "ghcversion.h") dirs)
+  found <- filterM doesFileExist candidates
   case found of
-      []    -> throwGhcExceptionIO (InstallationError ("ghcversion.h missing"))
+      []    -> throwGhcExceptionIO (InstallationError
+                                    ("ghcversion.h missing; tried: "
+                                      ++ intercalate ", " candidates))
       (x:_) -> return x
 
 -- Note [-fPIC for assembler]