SPS kernel skips inactive particles

In addition to commit d961b548, which introduced inactive check on
neibs
Fix to commit a454605c (correctly check BUFFER_BIG)

The bug propagated in many commits/branches but came out only when
re-testing multi-GPU SPS (which requires TAU bursts exchange)
make snapshot: we don't have .inc files anymore
Makefile: massage GPUSPH_VERSION and use it in snapshot filename
Introduce Makefile.conf and use it to store the compute capability
Move problem selection to new Makefile.conf mechanism
Move dbg selection to new Makefile.conf mechanism
Move fastmath selection to new Makefile.conf mechanism
Move hash key size selection to new Makefile.conf mechanism
Makefile comment fix
MPI usage is now managed like the other options
Do not pass --generate-line-info when compiling list-cuda-cc
Fix bug in handling of USE_MPI

When it wasn't explicitly set, and MPI was found, USE_MPI would not be
defined ever.
HDF5 usage is now managed like other options
make show: add new options
Include Makefile.conf before Makefile.local
Put a disclaimer on top of Makefile.conf
Compute capability detection: use runtime API

The code is simpler, and it doesn't require detection of the driver
library, which might be in any odd location on Ubuntu and Debian.