[O] (eval-when-compile (require 'async)) in org-download.el

[Oleh]

Hi all,

There's an issue (https://github.com/abo-abo/org-download/issues/7)
with `org-download` on some systems resulting from `async` not being
found even when the user has it on his system.

The problem comes from org's `make install` producing a different
org-download.elc than by manual byte-compilation.  The issue could be
resolved by putting (eval-when-compile (require 'async)) in
org-download.el, but `make install` does not see 'async as the paths
aren't set up when it does the compiling.

Any recommendations to resolve this issue are welcome.

Proposed variants include:
- setting `no-byte-compile` locally in org-download.el
- suggesting to use `(require 'org-download "org-download.el")` instead
  of `(require 'org-download)`

regards,
Oleh