RHEL 5-era autoconf did not define these, so define them manually
when they are missing.

Define builddir as '.'  The scripts require this.  It won't work
in the srcdir != builddir case, but we don't care about that for
RHEL 5.

This commit also moves the builddir / abs_srcdir variable setting
above the include of subdir-rules.mk, in case that include uses
these variables.

Useful script:

  for f in $(find -name Makefile.am | xargs fgrep '$(abs_srcdir)' -l) ; do
    if ! grep -q '^abs_srcdir' $f; then
      echo missing in $f
    fi
  done

We partitioned the disk, and then tried to create a PV on the whole
disk.  LVM gave the error:

  Device /dev/vda not found (or ignored by filtering).

It is unclear how this bug persisted for so long.  It might be due to
a change in LVM.

This large commit changes the generator so that optional arguments
can be supported for functions.

The model for arguments (known as the "style") is changed from
(ret, args) to (ret, args, optargs) where optargs is a more limited
list of arguments.

One function has been added which takes optional arguments, it is
"add-drive-opts", modelled as:

  (RErr, [String "filename"], #required
         [Bool "readonly"; String "format"; String "iface"]) #optional

Note that this function is processed in the library (does not go over
the RPC protocol to the daemon).  This has allowed us to simplify
the current implementation by omitting changes related to RPC or the
daemon, although we plan to add these at some point in the future.

From C this function can be called in 3 different ways as in these
examples:

  guestfs_add_drive_opts (g, filename,
                          GUESTFS_ADD_DRIVE_OPTS_READONLY, 1,
			  GUESTFS_ADD_DRIVE_OPTS_FORMAT, "raw",
                          -1);

(the argument(s) between 'filename' and '-1' are the optional ones).

  guestfs_add_drive_opts_va (g, filename, args);

where 'args' is a va_list.  This works like the first version.

  struct guestfs_add_drive_opts_argv optargs = {
    .bitmask = GUESTFS_ADD_DRIVE_OPTS_READONLY_BITMASK,
    .readonly = 1,
  }
  guestfs_add_drive_opts_argv (g, filename, &optargs);

This last form lets you construct lists of optional arguments, and
is used by guestfish and the language bindings.

In guestfish optional arguments are used like this:

  add-drive-opts filename readonly:true

In OCaml these are mapped naturally to OCaml optional arguments, eg:

  g#add_drive_opts ~readonly:true filename;

In Perl these are mapped to extra arguments, eg:

  $g->add_drive_opts ($filename, readonly => 1);

In Python these are mapped to optional arguments, eg:

  g.add_drive_opts ("file", readonly = 1, format = "qcow2")

In Ruby these are mapped to a final hash argument, eg:

  g.add_drive_opts("file", {})
  g.add_drive_opts("file", :readonly => 1)
  g.add_drive_opts("file", :readonly => 1, :iface => "virtio")

In PHP these are mapped to extra parameters.  This is not quite
accurate since you cannot omit arbitrary optional parameters, but
there's not much than can be done within the limitations of PHP
as a language.

Unimplemented in: Haskell, C#, Java.