gcc: leave setOutputFlags=false as it was before this PR

Too many downstream packages depend on the idiosyncracy of gcc's
custom `installPhase`.  Letting `make install` decide where things
belong (i.e. `setOutputFlags=true` so `configureFlags` gets things
like `--libdir=`) would impose too many changes on other parts of
nixpkgs.

The `patchelf` line added in this commit preserves the most
important thing that `setOutputFlags` was accomplishing: no
references from $lib to $out.

pkgs/development/compilers/gcc/11/default.nix

@@ -111,7 +111,7 @@ lib.pipe (stdenv.mkDerivation ({
   ++ lib.optional (!langJit) "lib"
   ++ lib.optionals enableLibGccOutput [ "libgcc" ]
   ++ lib.optionals enableChecksum [ "checksum" ];
-  setOutputFlags = enableExternalBootstrap;
+  setOutputFlags = false;
   NIX_NO_SELF_RPATH = true;
 
   libc_dev = stdenv.cc.libc_dev;
@@ -326,12 +326,18 @@ lib.pipe (stdenv.mkDerivation ({
       # in bash instead of nix (we should fix that).
       preFixupPhases = (previousAttrs.preFixupPhases or []) ++ [ "postPostInstallPhase" ];
       postPostInstallPhase = lib.optionalString langJit ''
+        # this is to keep clang happy
         mv $out/lib/gcc/${targetPlatform.config}/${version}/* $out/lib/gcc/
         rmdir $out/lib/gcc/${targetPlatform.config}/${version}
         rmdir $out/lib/gcc/${targetPlatform.config}
+      '' + lib.optionalString enableLibGccOutput ''
+        # eliminate false lib->out references
+        find $lib/lib/ -name \*.so\* -exec patchelf --shrink-rpath {} \; || true
       '' + lib.optionalString (!langC) ''
+        # delete extra/unused builds of libgcc_s to avoid potential confusion:
         rm -f $out/lib/libgcc_s.so*
       '' + lib.optionalString enableLibGccOutput (''
+        # move libgcc from lib to its own output (libgcc)
         mkdir -p $libgcc/lib
         mv    $lib/lib/libgcc_s.so       $libgcc/lib/
         mv    $lib/lib/libgcc_s.so.1     $libgcc/lib/