[SV 46013] Allow recursive variable overrides from Makefiles

[SV 46013] Allow recursive variable overrides from Makefiles

Ensure that variable overrides are passed to recursive make instances
even if no overrides were provided on the command line.
Fix suggested by Rici Lake <ricilake@gmail.com>

* src/main.c (define_makeflags): Add overrides without respect to the
value of command_variables.
* tests/scripts/features/recursion: Add a test.