rts: Don't declare libCffi as bundled when using system libffi

Previously the rts's cabal file would claim that it bundled libffi, even
if we are using the system's libffi. Fixes #19869.

(cherry picked from commit 63184a71)

configure.ac

@@ -1081,10 +1081,12 @@ AC_ARG_WITH([system-libffi],
 ])
 
 AS_IF([test "x$with_system_libffi" = "xyes"],
-  [UseSystemLibFFI="YES"], [UseSystemLibFFI="NO"]
+  [UseSystemLibFFI="YES"; CabalUseSystemLibFFI="True"],
+  [UseSystemLibFFI="NO"; CabalUseSystemLibFFI="True"]
 )
 
 AC_SUBST(UseSystemLibFFI)
+AC_SUBST(CabalUseSystemLibFFI)
 
 AC_ARG_WITH([ffi-includes],
 [AS_HELP_STRING([--with-ffi-includes=ARG],

hadrian/src/Settings/Packages.hs

@@ -245,6 +245,7 @@ rtsPackageArgs = package rts ? do
     way            <- getWay
     path           <- getBuildPath
     top            <- expr topDirectory
+    useSystemFfi   <- expr $ flag UseSystemFfi
     libffiName     <- expr libffiLibraryName
     ffiIncludeDir  <- getSetting FfiIncludeDir
     ffiLibraryDir  <- getSetting FfiLibDir
@@ -360,6 +361,7 @@ rtsPackageArgs = package rts ? do
           , any (wayUnit Debug) rtsWays     `cabalFlag` "debug"
           , any (wayUnit Logging) rtsWays   `cabalFlag` "logging"
           , any (wayUnit Dynamic) rtsWays   `cabalFlag` "dynamic"
+          , useSystemFfi                    `cabalFlag` "use-system-libffi"
           , useLibffiForAdjustors           `cabalFlag` "libffi-adjustors"
           , Debug `wayUnit` way             `cabalFlag` "find-ptr"
           ]

rts/rts.cabal.in

@@ -12,6 +12,8 @@ flag libdl
   default: @CabalHaveLibdl@
 flag ffi
   default: @CabalHaveLibffi@
+flag use-system-libffi
+  default: @CabalUseSystemLibFFI@
 flag libffi-adjustors
   default: @CabalLibffiAdjustors@
 flag need-pthread
@@ -57,10 +59,14 @@ library
     -- expects the unit-id to be
     -- set without version
     ghc-options: -this-unit-id rts
-    if os(windows)
-      extra-bundled-libraries: Cffi-6
-    else
-      extra-bundled-libraries: Cffi
+
+    -- If we are using an in-tree libffi then we must declare it as a bundled
+    -- library to ensure that Cabal installs it.
+    if !flag(use-system-libffi)
+      if os(windows)
+        extra-bundled-libraries: Cffi-6
+      else
+        extra-bundled-libraries: Cffi
 
     -- The make build system does something special in config.mk.in
     -- for generating profiled, debugged, etc builds of those