build: Remove extra targets from generate_databases.

These targets don't exist in this directory.
build: Fix rules for executables.

Since the executables were never mentioned by full path, they would be
rebuilt unconditionally.
Fix some of the simpler warnings.
Use MPI_STATUS_IGNORE in MPI calls.

The status is not used, so this saves us an extra variable or two. As
suggested by @jedbrown in another PR for Specfem2D.