This attempts to allow sharing a host directory across multiple builds, target
use case is to have a shared source directory on a VM host and be able to build
from that in a number of VMs without those builds stepping on top of each
other.
Instead of subsurface/build, subsurface/mobile-build,
subsurface/libdivecomputer/build, use a prefix path to allow having true out of
tree builds.
The one shortcoming is that the autotools need to be run in the libdivecomputer
directory - that means this will be run on the first system that starts a
build. But that seems to cause no harm in my testing.
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>