[SV 66037] Avoid hang/crash from MAKEFLAGS=... on command line

[SV 66037] Avoid hang/crash from MAKEFLAGS=... on command line

Make enters an infinite loop when some option and MAKEFLAGS=<value>
are specified on the command line.  For example,
    $ make -r MAKEFLAGS=hello=world

If decode_switches() runs handle_non_switch_argument() from within
the getopt() loop, it would recursively call decode_switches() to
enter a new getopt() loop, corrupting the state of the outer loop.

* src/main.c (decode_switches): Save up non-option arguments and run
handle_non_switch_argument() only after we're done with getopt().
* tests/scripts/variables/MAKEFLAGS: Add tests.