per-target clang configurations

[vekatze]

Before:

// module.ens
{
  target {
    foo "foo.nt"
  }
  // ...
}

neut build foo --clang-option "-fsanitize=address ..."

After:

// module.ens
{
  target {
    foo {
      main "foo.nt",
      build-option [
        "-fsanitize=address",
        "$(pkg-config libpq --libs --cflags)" // or whatever
      ],
    }
  }
  // ...
}

neut build foo