Add a stdenv adapter ‘keepDebugInfo’ to make a debug build

This adapter causes the resulting binaries to have debug info and no
optimisations.  Example use (in all-packages.nix):

  foo = callPackage ./foo.nix {
    stdenv = keepDebugInfo stdenv;
  };

pkgs/stdenv/adapters.nix

@@ -355,4 +355,16 @@ rec {
           drvPath = validate pkg.drvPath;
         };
     };
+
+
+  /* Modify a stdenv so that it produces debug builds; that is,
+     binaries have debug info, and compiler optimisations are
+     disabled. */
+  keepDebugInfo = stdenv: stdenv //
+    { mkDerivation = args: stdenv.mkDerivation (args // {
+        dontStrip = true;
+        NIX_CFLAGS_COMPILE = toString (args.NIX_CFLAGS_COMPILE or "") + " -g -O0";
+      });
+    };
+
 }