[SV 61218] Ensure MAKEFLAGS is expanded even with -e

[SV 61218] Ensure MAKEFLAGS is expanded even with -e

If -e was given we weren't expanding MAKEFLAGS before passing it
through the environment to jobs: we don't expand variables we
receive from the environment and when -e is given we set the
origin of MAKEFLAGS to "environment override".  Check for MAKEFLAGS
specifically, which seems like a hack but I don't have a better
idea offhand.

* src/main.c (main): Drive-by: use o_default for MAKEOVERRIDES.
* src/variable.c (target_environment): Always expand MAKEFLAGS
regardless of the origin type.
* tests/scripts/options/dash-e: Create a test.