hadrian: Only copy and install libffi headers when using in-tree libffi

When passed `--use-system-libffi` then we shouldn't copy and install the
headers from the system package. Instead the headers are expected to be
available as a runtime dependency on the users system.

Fixes #21485 #21487

(cherry picked from commit fec3e7aa)

hadrian/src/Rules/Generate.hs

@@ -40,13 +40,15 @@ rtsDependencies :: Expr [FilePath]
 rtsDependencies = do
     stage   <- getStage
     rtsPath <- expr (rtsBuildPath stage)
+    useSystemFfi <- expr (flag UseSystemFfi)
+
     let headers =
             [ "ghcautoconf.h", "ghcplatform.h"
             , "DerivedConstants.h"
             , "rts" -/- "EventTypes.h"
             , "rts" -/- "EventLogConstants.h"
             ]
-            ++ libffiHeaderFiles
+            ++ (if useSystemFfi then [] else libffiHeaderFiles)
     pure $ ((rtsPath -/- "include") -/-) <$> headers
 
 genapplyDependencies :: Expr [FilePath]

hadrian/src/Rules/Rts.hs

@@ -121,7 +121,7 @@ needRtsLibffiTargets stage = do
     let headers = fmap ((rtsPath -/- "include") -/-) libffiHeaderFiles
 
     if useSystemFfi
-    then return headers
+    then return []
     else do
         -- Need Libffi
         -- This returns the dynamic library files (in the Libffi build dir).

rts/rts.cabal.in

@@ -73,6 +73,9 @@ library
         extra-bundled-libraries: Cffi-6
       else
         extra-bundled-libraries: Cffi
+      install-includes: ffi.h ffitarget.h
+      -- ^ see Note [Packaging libffi headers] in
+      -- GHC.Driver.CodeOutput.
 
     -- The make build system does something special in config.mk.in
     -- for generating profiled, debugged, etc builds of those
@@ -163,7 +166,7 @@ library
     install-includes: Cmm.h HsFFI.h MachDeps.h Rts.h RtsAPI.h Stg.h
                       ghcautoconf.h ghcconfig.h ghcplatform.h ghcversion.h
                       -- ^ from include
-                      DerivedConstants.h ffi.h ffitarget.h
+                      DerivedConstants.h
                       rts/EventLogConstants.h
                       rts/EventTypes.h
                       -- ^ generated