Fortran: check for undeclared variables by forcing implicit none everywhere
C/C++: check for unused parameters and return types
removed unused variables from test cases
ci: do missing return and unused arg check with Github Actions
This further simplifies behavior to match the "build vs host" decision
we did with `c_args` vs `build_c_args`. The rules are now simply:
- `native: true` affects `native: true` targets
- `native: false` affects `native: false` targets
- No native flag is the same as `native: false`
I like this because you don't even have to know what "build" and "host"
mean to understand how it works, and it doesn't depend on whether the
overall build is cross or not.
Fixes#4933
Michael Hirsch, Ph.D
Michael Hirsch, Ph.D
John Ericson
Jussi Pakkanen