proposed patch for configure.in

Hi,

I'm proposing this patch to fix the warning about the non-existent
--enable-dynamic argument to configure. I also replaced old libtool
macros (may be it's too early) and diable static build by default. This
seems to work for me (unix build with dynamic libraries), but I would
like to be sure it does not break other builds before commiting.

Best regards,
Jean