[rfc] autoconf-refactor: change semantics of valgrind option?

Hi all,
  we currently have a non-standard way of handling valgrind: instead
of the common --enable-valgrind, we now use a --with-valgrind[=path].
I'd like to change that to the standard behavior of a boolean --enable
flag, the path can be passed using environment variables (CPPFLAGS).

This changes the user-visible behavior slightly, I'd like to gather feedback.

Thanks

-- 
    /kinkie