Don't record outputFile in the hashed flags

We don't want

    ghc --make M -o <file>

to force recompilation of all modules when <file> changes.  The -o
value is already taken into account by the recompilation machinery
when we check the modification time on the object file or the
executable.

compiler/iface/FlagChecker.hs

@@ -41,7 +41,9 @@ fingerprintDynFlags DynFlags{..} nameio =
         -- -i, -osuf, -hcsuf, -hisuf, -odir, -hidir, -stubdir, -o, -ohi
         paths = (map normalise importPaths,
                    [ objectSuf, hcSuf, hiSuf ],
-                   [ objectDir, hiDir, stubDir, outputFile, outputHi ])
+                   [ objectDir, hiDir, stubDir, outputHi ])
+                   -- NB. not outputFile, we don't want "ghc --make M -o <file>"
+                   -- to force recompilation when <file> changes.
 
         -- -fprof-auto etc.
         prof = if opt_SccProfilingOn then fromEnum profAuto else 0