Might be better to use a --with argument so that external builds can be located more easily in non-standard places

Grackle chemistry --> GEAR chemistry. Moved the output of subgrid model to a separate part of the HDF5 file.

We might hope to be clean against all the issues it identifies

Just uses abort() instead of exit() in non-MPI code

Removed old interaction debugging option and created a new one that is not associated with an SPH scheme.

Also detect whether the current POSIX implementation offers the optional function posix_fallocate(). If not, switch off the particle streamed logging.

Implement simple thread barriers to be used on systems where the POSIX implementation does not provide them. We default to the POSIX ones when detected at configure time.