Disable builtin variables and rules immediately, when -R or -r is
added to MAKEFLAGS inside the makefile.
* src/main.c (disable_builtins): Add new function disable_builtins().
(main): Call disable_builtins().
(reset_makeflags): Call disable_builtins().
* tests/scripts/options/dash-r: Add tests.
* tests/scripts/variables/MAKEFLAGS: Update tests.