* doc/make.texi (Options/Recursion): Clarify that MAKEFLAGS values
from the environment have precedence over those set in the makefile.
* tests/scripts/variables/MAKEFLAGS: Check boolean switches -k/-S,
-w/--no-print-directory and -s/--no-silent as follows:
1. A switch can be enabled or disabled on the command line.
2. A switch can be enabled or disabled in env.
3. A switch can be enabled or disabled in makefile.
4. Command line beats env and makefile.
5. Env beats makefile.
6. MAKEFLAGS contains each specified switch at parse and build time.
7. If switches are specified in multiple origins, MAKEFLAGS contains
the winning switch at parse and build time.
8. MAKEFLAGS does not contain the losing switch.
Also test that --debug settings from different origins are combined
together into one option.
* make.1: Document the new flag.
* doc/make.texi: Document the new flag. Remove suggestions that the
.SILENT special target is deprecated or should not be used.
* tests/scripts/options/dash-s: Test the -s and --no-silent options.
* NEWS: Add information about the new option.
