[SV 63347] Always add command line variable assignments to MAKEFLAGS

[SV 63347] Always add command line variable assignments to MAKEFLAGS

This commit introduces two visible changes:
1. Keep command line variable assignments in MAKEFLAGS at all times,
   even while parsing makefiles.
2. Define makeflags immediately when a makefile modifies MAKEFLAGS.

The new MAKEFLAGS and MAKEOVERRIDES initialization procedure:
1. decode_switches (argc, argv, o_command) is called to parse command
   line variable assignments.
2. Command line variable assignments go through quote_for_env.
   Initialize -*-command-variables-*- to the quoted values.
3. MAKEOVERRIDES is initialized to refer to -*-command-variables-*-
   with origin o_env to keep the definitions in the database intact.
4. define_makeflags() is called which adds MAKEOVERRIDES to MAKEFLAGS.
5. Makefiles are parsed.  If a makefile modifies MAKEFLAGS, the new
   value of MAKEFLAGS is defined right away.
6. Env switches are decoded again as o_env.  The definitions set by
   decode_switches at step 1 stay intact, as o_command beats o_env.

We must preserve the original intact definitions in order to detect
failure cases; for example:
  $ cat makefile
  all:; $(hello)
  $ make hello='$(world'
  makefile:1: *** unterminated variable reference.  Stop.

* src/makeint.h: Declare enum variable_origin, struct variable and
define_makeflags().  Add parameter origin to decode_env_switches().
* src/main.c (define_makeflags): Remove "all". If a variable is
assigned on the command line then append MAKEOVERRIDES to MAKEFLAGS.
(decode_env_switches): Replace parameter env with origin.
(decode_switches): Replace parameter env with origin.
Treat origin == o_command as env == 0.
(handle_non_switch_argument): Replace parameter env with origin.
Treat origin == o_command as env == 0.
(main): Call decode_switches() with origin==o_command before parsing
makefiles.  Call decode_switches() with origin==o_env after parsing
makefiles.
* src/variable.c (set_special_var): Define makeflags at parse time,
each time a makefile modifies MAKEFLAGS.
(do_variable_definition): Strip command line variable assignments from
MAKEFLAGS before appending extra flags.  set_special_var() adds them
back.
* tests/scripts/variables/MAKEFLAGS: Add tests.