Add MPI phase_chain example, fix bug in phase_chain_omp_state.
Use Jam rule to only build OpenMP targets with supported compilers.
Revert "doc: define blocks for examples"

Using a simpler example now.

This reverts commit 12987f8f51594e7d6af8a0ec32485c9c6e137be2.
doc: add phase_chain examples to table
doc: use phase_chain example for MPI too