gnugrep: enable parallel build and tests

On a 16-core system number changes are:

- before: 1m34s
- after: 39s

2.5x speedup. If ./configure phase was faster the change would be even
more substantial.

pkgs/tools/text/gnugrep/default.nix

@@ -36,6 +36,8 @@ stdenv.mkDerivation {
     export MKDIR_P="mkdir -p"
   '';
 
+  enableParallelBuilding = true;
+
   # Fix reference to sh in bootstrap-tools, and invoke grep via
   # absolute path rather than looking at argv[0].
   postInstall =