This bumps subsurface to 4.9.3.
As the subsurface-specific libdivecomputer source is shipped as a git
submodule these days, we can avoid having to fetch a separate tarball
that needs to be kept in sync (and is only uploaded for each release).
This also applies a patch on top of 4.9.3 to properly handle libgit2
versions 1.0 and later - conditional code for older libgit versions has
been removed upstream, but that's a nontrivial backport, which is why
this patch was added.
We do a few more things:
- libmarble and libdivecomputer with the subsurface patches have both been
inlined for 2 reasons:
a) nobody else is using these forks
b) they need to be updated in lockstep with subsurface
- instead of building libmarble against qtquick1, we now build using
qtquickcontrols (qtquick2)
- getting rid of qtquick1 also allows us to use the default qt (currently 5.6)
- we get rid of some hacks to deal with library dependencies
- instead of manually overriding various phases, we simply use cmakeFlags to
pass proper parameters
