updates to libdc with ABI change

[Dirk Hohndel]

Next time you pull to build from source, please make sure you pull BOTH
Subsurface and the Subsurface-branch of libdivecomputer, otherwise you'll
lose functionality (most importantly, native BT support).

Linus has rewritten our custom serial handling into a custom io layer
which deals with packetization as well.

I took the opportunity to warn when building against an older version of
our branch or against upstream libdivecomputer.

Let us know if this broke things in more ways than we anticipated :-)

/D