Make some targets always serial

• Add some special handling for always building a serial version of mp

• Add tools for making targets serial and apply to ingen and other serial codes

• Tweak method for adding git_hash.o to dependencies for consistency

• Move fft_save_wisdom handling to target makefile and use consistent treatment

• Update utils commit to fix mp serial build

Adds some custom rules such that we can always build a serial version of the mp module alongside the parallel version if we wish. This allows us to switch to a serial version of a code by relinking with mp.o switched for mp_serial.o.

Extra makefile changes are then introduced for always making certain codes serial. This includes ingen. The motivation was to make it easy to always get ingen built suitable for running on the front end of HPC where sometimes jobs which initialise mpi are not allowed.

Some additional makefile tweaks to adopt the same approach to augmenting dependency lists which should be easier to add new codes to.