When running on valgrind, don't chroot or drop privileges

When running on valgrind, don't chroot or drop privileges

Valgrind documentation says:

  You are encouraged to copy the valgrind/*.h headers into your
  project's include directory, so your program doesn't have a
  compile-time dependency on Valgrind being installed. The Valgrind
  headers, unlike most of the rest of the code, are under a BSD-style
  license so you may include them without worrying about license
  incompatibility.

I don't like copying headers around. Therefore, there is an autoconf
check for valgrind headers.