Import bmake-20180512

Of relevance:

o job.c: skip polling job token pipe
o parse.c: be more cautious about detecting depenency line
  rather than sysV style include.

also in mk:

• dirdeps.mk: include local.dirdeps-build.mk when .MAKE.LEVEL > 0 ie. we are building something.
• FILES: add dirdeps-options.mk to deal with optional DIRDEPS.
• ldorder.mk: describe how to use LDORDER_EXTERN_BARRIER if needed.