package/beecrypt: fix C++ support

C++ support unconditionally needs shared libraries; it uses
dlopen/dlsym.

Also, fix the conditions under which the comment is shown.

Finally, explicitly require C++ support when configuring.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>