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 variable parsing detects more errors.
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.
Key changes include reduced noise at end of failed build log
and avoid evaluation of unnecessary terms in conditionals.
In META MODE; a target flagged .META is out-of-date if meta file
is missing
MFC after: 1 week
