Relevant/interesting changes:

	o parse.c: don't print null filename in stack traces
	o for.c: skip syntactically wrong .for loops
	o var.c: allow for :gmtime=${mtime}
	add :mtime[=timestamp] where timestamp is used if stat(2)
	fails, if :mtime=error stat(2) failure causes error.
	o make.1: fix documentation of .PREFIX to match reality and POSIX
	o unit-tests: improved var-scope-local

o allow env var MAKE_OBJDIR_CHECK_WRITABLE=no to skip writable
  checks in InitObjdir.  Explicit .OBJDIR target always allows
  read-only directory.

o Fix building and unit-tests on non-BSD.

o More code cleanup and refactoring.

o More unit tests

Lots of new unit-tests increase code coverage.

Lots of refactoring, cleanup and simlpification to reduce
code size.

Fixes for Bug 223564 and 245807

Updates to dirdeps.mk and meta2deps.py

Lots of code refactoring, simplification and cleanup.
Lots of new unit-tests providing much higher code coverage.
All courtesy of rillig at netbsd.

Other significant changes:

o new read-only variable .SHELL which provides the path of the shell
  used to run scripts (as defined by  the .SHELL target).

o new debug option -dl: LINT mode, does the equivalent of := for all
  variable assignments so that file and line number are reported for
  variable parse errors.

Changes since 20181221 are mostly portability related
hence the large gap in versions imported.

There are however some bug fixes, and a rework of filemon handling.
In NetBSD make/filemon/filemon_ktrace.c allows use of fktrace
and elimination of filemon(4) which has not had the TLC it needs.

FreeBSD filemon(4) is in much better shape, so bmake/filemon/filemon_dev.c
allows use of that, with a bit less overhead than the ktrace model.

Summary of changes from ChangeLog

	o str.c: empty string does not match % pattern
	  plus unit-test changes
	o var.c: import handling of old sysV style modifier using '%'
	o str.c: refactor brk_string
	o meta.c: meta_oodate, CHECK_VALID_META is too aggressive for CMD
	  a blank command is perfectly valid.
	o meta.c: meta_oodate, check for corrupted meta file
	  earlier and more often.
	* meta.c: meta_compat_parent check for USE_FILEMON
	  patch from Soeren Tempel
	o meta.c: fix compat mode, need to call meta_job_output()
	o job.c: extra fds for meta mode not needed if using filemon_dev
	o meta.c: avoid passing NULL to filemon_*() when meta_needed()
	  returns FALSE.
	o filemon/filemon_{dev,ktrace}.c: allow selection of
	  filemon implementation.  filemon_dev.c uses the kernel module
	  while filemon_ktrace.c leverages the fktrace api available in
	  NetBSD.  filemon_ktrace.c can hopefully form the basis for
	  adding support for other tracing mechanisms such as strace on
	  Linux.
	o meta.c: when target is out-of-date per normal make rules
	  record value of .OODATE in meta file.
	o parse.c: don't pass NULL to realpath(3)
	  some versions cannot handle it.
	o parse.c: ParseDoDependency: free paths rather than assert

plus more unit-tests