bug fix; solve bug introduced by r11524 and reported by totoazero and xqt

Externals: add support for other platforms than windows/linux
- Any OS without explicit support will now trigger download_install
- SunOS now has explicit non-support (i.e. a function that will trigger download_install)

Change-Id: I30c0efab5cb2c3dfc0cb574792e7a17a5a00e2a3