Patch adapted from Sage. Allows building ECL on libffi 3.3.
diff --git a/src/c/ffi.d b/src/c/ffi.d
index 8861303e..8a959c23 100644
--- a/src/c/ffi.d
+++ b/src/c/ffi.d
@@ -133,8 +133,8 @@ static struct {
 #elif defined(X86_WIN64)
   {@':win64', FFI_WIN64},
 #elif defined(X86_ANY) || defined(X86) || defined(X86_64)
-  {@':cdecl', FFI_SYSV},
-  {@':sysv', FFI_SYSV},
+  {@':cdecl', FFI_UNIX64},
+  {@':sysv', FFI_UNIX64},
   {@':unix64', FFI_UNIX64},
 #endif
 };